Abstract: The b value is estimated from the statistics of actual earthquake data. It depends on the completeness and reliabilty of the data,the sampling size,sampling range in time and space,the threshold magnitude of earhquakes and the sampling interval,etc.In this paper,we use the relationship lgN = a-bM between the accumulative frequency and magnitude. In order to search for the after effects of various factors on the result of b value estimation,we estimate the b value with various thresholdmagnitude and magnitude interval for different temporal and spatial ranges respectively. We found some way for reasonably obtaining the b value with experimental basis.In addition,we discuss the problem of using historical data and recent observed data simultaneously and investigated also the influence of the 'bending' at both ends of the linear relationship,lgN = a - bM on b value estimation. Then we suggest the principle and method of choosing a reasonable b value in seismic hazard analysis. Finally,we discuss the error range in estimating the b value and its impact on hazard analysis.