The concept of system-of-systems contribution is studied. To overcome the shortcomings of current methods for system contribution evaluation, the evaluation index system of the system contribution of space-based weapons is developed through the study on offensive and defensive capabilities of space-based weapons and their influence on the operation results. The quantitative mathematical model is built for the index with clear mathematical meanings. An evidential reasoning method based on fuzzy synthesis is proposed to evaluate the index with uncertain mathematical meanings. A typical space-based weapon combat system is developed, and the combat process of space-based weapons is analyzed by simulation for different operation modes, thus to evaluate the contribution of the weapons. The simulation results show that the index system for the contribution of space-based weapons is effective. It can successfully evaluate the system contribution of space-based weapons for a given combat system, and the result is reliable.