Abstract: Abstract: Extending the interest rate immunization theory, this article employs the mortality immunization theory to study the natural hedging strategy of longevity risks facing life insurance companies. Firstly, we give the definition of mortality duration and mortality convexity, their computation methods ,and then explore several factors influencing policy’s duration and convexity. On the basis of the above knowledge, we give the minimum ρ ratio between life policies and annuity policies in order to get the perfect hedging, besides ,we studied the nature of this ratio.