Cylinder deactivation for automotive engine, to decrease the fuel consumption, has an adverse effect on power-output of engine. The multi-body model of the crank-rod mechanism was created. Based on pressure-volume diagrams and empirical equation, the boundary conditions of the model were analyzed. Three cylinder deactivation modes were given. The simulation under the whole cylinder operation mode and the cylinder deactivation modes were investigated at different load conditions. The results showed that the engine speed fluctuations under the deactivation mode differ from the fluctuations under the whole cylinder operation mode, with respect to operating cylinder load and the firing interval angle.