The precision programmable fused silica capacitor plays an important role in the precision transmission of quantum capacitance and the calibration of capacitance measuring instruments, the output of any capacitance value in the range can be realized by controlling the opening and closing of each small capacitor unit through the coaxial switch. However, due to the influence of parasitic capacitance, the actual capacitance value is greater than the nominal value, and the smaller the capacitance unit, the greater the error. Based on Ansoft Maxwell, the precision programmable capacitance small capacitance unit is carried on thorough the research and analysis in this paper. According to the simulation results, the design scheme are confirmed, the actual design and manufacture of precision programmable small capacitance units are decided.