Abstract: With the new life table of 2000-2003 publication, life insurance pays more attention to the mortality rate. For example, considering the mortality rate development trends and its effect on life insurance in the future. So we should study more about it. To this paper, in the first section, specific background is introduced. As to mortality force, its function are estimated by least square method and credibility test is given in the second section. At the same time, in the third section, it shows the relationship between and, which reflects the mortality force change with time lapse under the life table of 1990-1993、2000-2003. Moreover, generally, the concept of random variable about Brownian motion to the mortality force formula. Also, the parameter estimated and test given as that of section two. Fourthly, it forecast the effect of decreasing mortality trends on net premium of annuity, providing the concrete example to analyze the longevity risk. Lastly, relative remark and recommendation are given.