Capability planning of weapon system of systems(WSoS) is an important part of the WSoS research area. The number of troops that can be dispatched and the ability to carry weapons are limited in mission fulfillment. Therefore, a capability planning method based on the effectiveness value of WSoS is proposed. Using the method, the capacity planning solutions for troops of different sizes can be quickly found by considering the capability requirement of various tasks in combat, and the relationship between the weapon equipment supporting different capabilities and their number. Focusing on the missions and analysing their capability requirement, a normalized combat capability effectiveness model is established firstly. Then, based on prospect theory, the effectiveness function of combat capability considering risk factors is proposed. Furthermore, the WSoS capability planning model based on effectiveness value are built under the constrain of risks and number of weapon units, the capability planning solution can be obtained with the maximum effectiveness value of WSoS. Finally, armed police force's anti-terrorism mission is analyzed as a example to verify the effectiveness and feasibility of the proposed method.