At present, excessive balance is used for the balance of traditional single cylinder engine. Single shaft balance may also be used on single cylinder engine with large power and high rotation speed. In this article, a small single cylinder engine 250 is studied. By our research, the mass centre of excessive balance block is not at one straight line with the central line of cylinder to result in heavier horizontal vibration of whole machine. In this article, a modification scheme is offered. In this scheme, for more reduction of vibration, a balance device for single shaft is designed for machine, and find out reasonable installation place by optimization method such that the impact of additional torque, arising from the balance of single shaft, to vibration of whole machine may be reduced to min level to realize balance optimization of the machine.