To improve the efficiency of force optimization algorithms, a new method to compute initial grasping force is proposed. A group of grasp forces with respect to external unit forces separately is computed, and an arbitrary external force is decomposed into linear combination of unit forces. The initial force is obtained by linear combination with the same rule. An example indicates that the new method reduces the steps of convergence of force optimization algorithms and is faster than Lagrange dual method and single value optimizing method. The method can be used to provide initial values for the force optimization algorithms in the point contact friction models, and improve efficiency.