For the purpose of evaluating the effectiveness of space weapon system, this paper firstly analyzes the structure of space weapon system, then the guide line of effectiveness evaluation of space weapon system is built. Aimed at the characteristic that the information of space weapon system is partly known and partly unknown, an improved demarcating matrix method of determining the weigh is put forward, a new evaluation model which combines grey theory with analytic hierarchy process for effectiveness evaluation is presented. Finally, an example of the assessment model is calculated and the result shows that the model is reasonable, feasible and effective.