This paper describes a decoupling force control strategy of redundant manipulator by workspace observer. In general, the calculation process of the redundant manipulator becomes complex as the number of degree-offreedom motion increases because the controller includes the pseudo inverse Jacobian matrix. To alleviate this problem, we develop a simplification method of the inverse kinematics solution including null space calculation by using the transposed Jacobian matrix, and the workspace observer. Here the stable condition of the proposed controller is also presented. Its condition is an important index to construct the stable workspace observer.