| This thesis consists of two parts: In part one, we discuss circumstellarenvelopes of AGB stars, and study the outflow velocity in envelope. Themaser variability is discussed in part two. The results of observations ofwater masers are presented.During the AGB phase the stars begin to expel matter at a high rate(10-7-l0(-4)M?/yr). The process 1asts for about l05yr and builds acircumstellar envelope which is a good testing ground for ideas and theoriesabout astrophysical progresses: mass loss, maser excitation and radiativetransfer; formation of solid state particles; molecular chemistry; radiationdriven hydrodynamics. Hence, it is very important for star evolution fromAGB to PN.In order to study the velocity fields of circumstellar envelopes, amodel of an axial-symmetrically rotating and expanding envelope of AGBstars has been put up. The results show that the mass-loss rate enhancedtowards the equatorial plane and the outflow velocities increasemonotonously with latitude above equatorial plane and reach maximum inthe pole.The time variation is one of the most important features ofastronomical masers, which shows the variability of the peak line intensity,the line width at the half power points, and the line center velocity and thetime scales range from yeas to a few days. Its detailed study is an importantprobe in the dynamics of the star-forming regions, circumstellar envelopesand active galactic nuclei. We report the observing results of a short timescale monitoring of the 22 GHz water maser emission in W3(OH) andNGC6334C.The timescales of 19 and 8 day have been detected, and thevariations are also accompanied by some changes in the linewidth. The timevariations were probably caused by a collisional pump which led to changein the pump input.
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