Traditional method for positioning the location of work-piece requires that the internal and external parameters of camera must be first calibrated. Based on the idea of "black box" that directly mapping the coordinate of camera to robot coordinate. A constant rotation matrix that maps end-effector coordinate to robot coordinate is proposed in this paper. The constant rotation matrix simplifies the algorithm and process of object 3-D position, and it can achieve high accuracy. The method can also implement object position when the end-effector changes its position. The method for object position can also be used in mobile assembly robot and mobile operator.