In order to solve the problem that it is difficult to calculate the average efficiency area accurately because of the diversity and randomness of ground targets, the lethality evaluation method of blast-fragmentation warhead based on physical lethality area is proposed, which is used as the basis for determining the optimal blast height of blast-fragmentation warhead. According to the given terminal ballistic parameters and fragmentation parameters of blast-fragmentation warhead, the calculation model of physical damage area is established, the calculation program is compiled, and the maximum physical damage area is taken as the condition to determine the burst height. The results show that the impact velocity and impact angle of the missile have great influence on the optimal height of the blast-fragmentation warhead. This method can be used as the basis for determining the optimal burst height of the blast-fragmentation warhead and optimizing the fragment parameters.