A unified approach to solving driving forces in spatial parallel manipulators with less than six DOFs is proposed. First, the geometric constrained equations of the parallel manipulators are derived, and some independent pose parameters and a common transformation matrix with three translations and three Euler rotations are determined. Second, the common formulas for solving inverse kinematics, the Jacobian matrix, and velocity are derived. Third, a common virtual serial mechanism with three virtual prismatic joints and three virtual revolute joints corresponding to three Euler rotations is constructed. Fourth, a common analytic formula for solving driving forces in spatial parallel manipulators with less than six DOFs is derived by using the principle of virtual work and the virtual serial mechanism. Finally, a 3-SPR parallel manipulator with three DOFs and a 4SPS+SPR parallel manipulator with five DOFs are presented to illustrate the use of the unified approach to solve their inverse kinematics and the driving forces. The solving results are verified by the simulation mechanisms.