With the instruction of computational dynamics of multi-body systems theory, considering factor of the crankshaft system with variable inertia, the torsional vibration model of crankshaft system was established. Based on kinetic energy equivalence principle, the variable inertia of crankshaft system was analyzed by computational dynamics of multi-body systems method; and the characteristic relation curve between torsional vibration frequency and crank angle was attained; then the belt type distribution characteristic of critical speed was obtained. The analysis considering the nonlinear factor of variable inertia was complicated, but the result was much more approach to the fact. The whole analysis laid the foundations of diesel structure design, and provided a simple and feasible approach to torsional vibration analysis with variable inertia of other systems.