The computation principle of Cockroft-Lathem ductile cumulative damage is analyzed. A method to solve the critical damage factor based on the evolution rules of damage sensitive rate is brought forward. Several series of billet samples are compressed on heat physical simulation machine under different deformation temperatures and strain rates, and then the collected true stress-strain data are used to calculate the performance of computations. As the outer edge of upsetting drum where the maximum damage value appears is concerned, a typical damage softening rule is obtained and described as the damage sensitive rate decreases rapidly at the initial deformation, then gradually tends to 0. In further, damage softening is more sensitive to strain rate than temperature. A principle that critical damage factor arises when damage sensitive rate decreases to 0 is assumed, then the distribution of critical damage factor under temperature 573～723 K and strain rate 0.01～10 s-1 is obtained. The results show that the critical damage factor of 7075 aluminum alloy is not a constant but a change within a range of 0.255 ~ 0.453, and it is more sensitive to strain rate in high temperature and more sensitive to temperature in high strain rate.