Based on the requirement of fixed accuracy and small structure in metering industry, a structure design method is given. The ideal curve of metering structure under certain constrains is derived through theory computation, then the correctness of such method is proved. Then, three approximation methods of the ideal curve are provided to meet the requirement of metering accuracy. At the same time, the relationship between traditional metering structure and the three approximations is described. Together with the consideration of structural manufacturability, the optimum of the approximation is recommended. Finally, based on the design requirement of a certain fuel metering unit, a case of metering structure design is implemented using the optimum approximation method.