The effect of piston structure and size on the power and economy performance of the engine was analyzed． The piston structure of the X195 diesel engine was reformed and 3 types reformed piston were used to make testbed test. The test was made with in the same condition except to change the piston. The results showed by changing that the original design of 4 pistonring slots to 3 ones， changing 3 gas rings into 2 ones,increasing the top diameter of the piston, informing the compensating slot, reducing the length of the piston skirt, and changing the step form of the piston skirt into taper, the friction area can be reduced, the lubricate condition of the piston can be ameliorated, the frictional loss was effectively reduced, so that the maximum output power of the engine can go up 4.57 %, and the rate of fuel consumption can reduce by 9.34 %.