Aiming at the problems of complex structure, large number of parts and low assembly efficiency in ammunition assembly, a method of generating ammunition assembly sequence planning based on model based definition (MBD) was proposed. On the basis of defining the feasibility of assembly sequence, the fitness function of assembly planning is constructed, and an assembly sequence planning algorithm based on improved genetic algorithm (GA) including elite population is proposed. Based on the secondary development technology of UG/NX, the assembly sequence planning system of MBD is designed and developed. A fuze sub-assembly is taken as an example to verify the effectiveness and stability of the proposed method.