| The 8th compulsory educational reform advocates science inquiry-oriented teaching & learning. During the course of the activities of the science inquiry-oriented learning, it is very important to know the existing cognitive level of the children for the design of the teaching & learning activities. The process of learning science is the course of the development and the transformation of preconception on which children based to understand the nature, it studies students' understanding and learning of science conception, analyzes the causations of the preconception, the course of the developing and the transformation of the preconception, these are all beneficial to improving the science teaching, and can also provide valuable advice for compiling the science teaching materials.The conception of the buoyancy is one of the important points of physics teaching in secondary school, and also a point that bewilders teachers and the students so much. Since student's original understanding of the buoyancy begins with the phenomena of the floating and sinking, This study based and improve the method of clinic interview and designed a program of "classification of floating objects and the sinking objects ", to research students who ranged from the 2nd grade, 3rd grade to 5th grade and 7th grade without a systematical knowledge of buoyancy in science class, how to understand the phenomena of floating & sinking and their causations.The conclusion based on the student's strategies for analyzing the object's circumstance of floating & sinking showed that the strategies that the student utilized mainly are: the strategy of materials, light or heavy, hollow or solid, its density of pressure, of sustain and of moving and so on. The strategies students utilized most are as follows: the material strategy, light or heavy, hollow or solid, pressure and sustain strategy. The percentage of light or heavy strategy decreases remarkably, while the percentage of the density strategy increases dramatically. The percentage of the material strategy, strategy of hollow or solid used by 2nd, 3rd and 5th years students increases much faster than those coming from 5th grade to 7th grade, while the pressure and sustain strategies are frequently used by all the students of three grades. The percentage of the pressure strategy declines slowly while the strategy of sustain fluctuates a little. According to the students' analysis, we can draw a conclusion that they classify the objects into 3 types: the objects that are hollow or solid, the ones that cannot be ascertained what the material is from the appearance and the ones that their areas and shapes can be changed. We can also conclude that these analyses are uncertain and vague.Based on these factors, we can divide the student's cognitive development of comprehension on floating & sinking into 3 stages. Students who are in the first stage analyze the object's floating & sinking mainly by their weight or their hollows. They cannot divorce the floating & sinking from the appearance characteristics such as the weight, size and hollow etc , so they have no density strategy. With the increase of age, the percentage of students located in this stage decreases considerably. Students in the second stage begin to take the characteristics of materials into account and know how to deal with the dialectic relationship between the floating & sinking and the weight. They begin to know it is wrong to attribute floating and sinking to their weight or to see whether they are hollow or not. Therefore they can divide objects into four types: big and heavy, big but light, small while heavy and small and light. But they can also not coordinate the relationship between floating & sinking and the weight or whether they are hollow or not especially that those objects are big but light or small but heavy. The percentage of all the three grades students does not change much and are all in high level. Students in the third stage can analyze the object's floating & sinking situation from the view of their...
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