Abstract: Balance of engine’s exciting vibration sources while the engine normally vibrated was analyzed, the engine vibrating forms on the frame were discussed. The results showed that the overturn moment is the main exciting vibration source which will change when individual cylinder power changes. Because of overbalanced cylinder power, every cylinder’s nonmain harmonic overturn moments (appearing in many harmonics and uniting together now) should offset each other which lead to worsen the balance of the gross overturn moment, and aggravate engine vibrating. Finally,an engine was experimented by the method of the vibration analysis.