After the tunnels are excavated, the original state of surrounding rock is destroyed. If the bearing capability of surrounding rock is not enough, the surrounding rock may be damaged and the whole underground rock mass lose even its stability. In the case, the primary support must be installed. But there is still lack of the accurate calculation about support parameters, so the empirical method is always relied on, which leads to the unsafe and uneconomical support. Based on MohrCoulomb criterion, the coupling interaction of primary support and surrounding rock is studied, and the more safe and economical design method is presented in this paper.