In this paper, based on the micro-prism coupling of side pump, the fiber is wound into a coil around a object of racetrack shape to make the micro-prism couple the pump light in many positions. Based on population rate equation, pump and signal propagation equation, a numerical simulation has been done. In the case of end pump and side pump, pump power of different position is same and the hole pump power is same, the fiber gain change with the fiber length has been discussed. It is found that the side pump coupling in many positions increases the fiber gain. The influence of the bending loss and the residual pump power on the fiber gain has been discussed too, the result showed that the fiber bending can′t be too small and the residual pump power can be neglected. In this way, it can not only offer a simple way to design the micro prism and operate conveniently, but increase the signal output gain effectively.