The structure of plunger of the in line fuel pump diaphragm for dimethyl ether(DME)engine was designed.The plunger clearance leakage characteristics were analyzed by AMESim,the impact of the clearance size,the fit surface length and the plunger diameter on the instantaneous leakage was discussed.The results show that,with the increases of clearance size,the instantaneous leakage at the same time is increased,the increasing rate of leakage is also gradually raised.The leakage is increased with the increase of plunger diameter,and the fit surface length has little impact on the leakage.By optimizing the design,the reasonable structure parameter range of the plunger is got,the design projects of leakproof plunger and low pressure recovery path are proposed,and the structure of the in line fuel pump diaphragm is further improved.