Coherence is an important parameter, which appraises quality of topical interferometery fringe and reflects the volume scatter information in InSAR. So it is important for generation of DEM, classification of surface features, estimation of backscatter layers, and quality of differential interferometery processing. In this paper, all kinds of factors, such as spatial and temporal decorrelation, thermal decorrelation, coregistraion decorrelation, Doppler decorrelation, influences on the coherence during InSAR processing are expatiated. Based on the practice of interferometery processing to 28 SAR images coming from Ms8.1 serious earthquake area which located in KUNLUN mountains West-pass bordering QINGHAI and XIZANG province in 14th Dec, 2001; and 5 SAR images coming from Ms7.9 serious earthquake area which located in Tibet North MANI region in 8th Nov., 1997; and 8 cog-nominal ERS-1/2 SAR images of environmental investigation on DATONG Jurassic & Carboniferous period coalfield subsidence by InSAR technology during 1992 to 2003, and influencing degree of these factors on InSAR processing are analyzed. Moreover, laws of decorrelation are concluded and some suggestions for reducing the decorrelation are brought forward.