An internal leakage model for the flowmeter of piston type was established. Formulas of relative measuring accuracy were deduced when the leakage flow regime was laminar. For the measuring accuracy model of flowmeter, the relations of changing between relative measuring error and its influence factors were investigated. Finally, the piston clearance is optimized in theory to get the scope of design value based on simulation results of measuring accuracy and all influence factors, which will improve the measuring accuracy of flow meter.