Abstract: In this paper, based on Reference 7, a relatively detailed study is made on the nature of maximum likelihood estimated value b by mumerical simulation.The result shows that in addition to magnitude range (M2-M1) and graded interval △M, the size of real value b itself also has a certain effect on deviation of estimated value b.It is pointed out that b is a consistent, asymptotic effective and asymptotic non-deviatoric estimated value of b, and the convergence rate is rather fast.For real value b=0.80, at the statistical earthquake number N=20,the deviation and mean square error of estimated value b are basically stabilized at about+0.04 and 0.20.So in the areas Where earthquake frequencies are not high, it is reasonable to make routine measurement of b value by maximum likelihood method.