| Levelness of the module is the key index during the lifting construction, under most conditions, counterweight is needed to meet the levelness requirement. If we use classical FEM for lifting analysis, additional constraints are always needed to eliminate the rigid body displacement, moreover, if we want to confirm the counterweight, we may need repetitive compute the balance equations, the computational efficiency is still unsatisfactory. To overcome these problems, the motion of the lifting module can be decomposed into the overall motion of the body reference and the elastic motion with respect to the body reference. Hence, the under-constrained problem can be converted to a static equilibrium problems of a multibody systems. In the following, the node force balance equations and its tangent stiffness matrix are obtained based on the virtual power principle. Then, the counterweight is expressed as the function of the basic counterweight and the load factor. The balance equations can then be converted to a set of ordinary differential equation (ODE), with efficient algorithm of the ODE, the counterweight can be easily and quickly obtained. Numerical examples shows that the proposed method can be a new approach for under-constrained problems and with reasonable accuracy and relatively rapid speed to obtain the counterweight.