Abstract: In Monte Carlo simulation,the α eigen values are usually calculated using the（k,α） iteration method.This method can give good results for supercritical and light subcritical systems,but due to the limitation of this method,it can not give the right results of α eigen values in deep subcritical systems.To calculate the α eigen values in deep subcritical systems using Monte Carlo method,a method named Transient Curve Fitting method which is based on the MC transient code TMCC is put forward.The Transient Curve Fitting method can solve the difficulties of simulating the α eigen functions and calculate the α eigen values effectively.In this paper,the results of this method are compared with MCNP results by calculating super-critical and subcritical problems,and they agree well with each other.And then the results of this method are compared with the experimental results of Subcritical Facility in Tsinghua University.And the results represent that the transient curve fitting method can calculate the α eigen values of reactor systems,and can give accurate results in deep subcritical situations.