In many space applications, the spacecraft (SC) must have good agility performance, which depends heavily on the capability of attitude control system. This paper aims to maximize the onboard capability of SC attitude control system by optimizing the use of reaction wheels (RWs). The authors firstly investigated the optimal configuration of the rotation axes relative to cluster design frame, and the cluster arrangement relative to the SC body frame. Then, the octahedron pyramid configuration was selected as the RWs configuration. For this configuration, the cluster of two shifted assemblies (four wheels each) has a 20.7 % larger envelope volume, and a 10 % longer inscribed sphere radius than the cluster of coinciding assemblies. Using the optimal agility performance criterion, the cluster of shifted assemblies can maximize the system capability by increasing the SC acceleration by 9.85 % along the worst direction. Subsequently, the controller saturation limits were updated depending based on the number and arrangement of the RWs. In case of one RW off, the SC acceleration in roll or pitch channel could be enhanced by 26.23 %. Overall, our RWs configuration could enhance the SC agility by 38.51 %. The research findings make it possible to optimize the agility of the SC and rationalize the selection and sizing of the RWs.