Compared with dexterous grasps, power grasps can support larger external loads and grasp objects more steadily. Because there are more contact constraints between the object and the grasping mechanisms, and the contacts may occur on the parts with limited mobility of the grasping mechanisms, the proposed methods which are applicable to the contact force distribution for dexterous grasps can not be used to distribute the contact forces of power grasps. In this paper, based on the analysis of the structural characteristics of a type of power grasping mechanisms, the object and contact coordinates are suitably established. And then, the general formula of contact distribution for a type of power grasps is put forward through the decomposition of the contact force space. From the obtained formula, the function relation among the external loads, joint torques and contact forces is also set up.