In the programmable capacitor, due to the influence of stray capacitance generated by the edge effects, it is difficult to achieve accurate and stable small capacitance standards of subpF and below. In this paper, a method of implementing a small capacitance standards based on Kelvin equipotential protection electrode is proposed, so that the stray capacitance affecting the small capacitance is limited to the stray capacitance generated by the edge effect between the center electrode and the guard electrode. The analytical model of equivalent small capacitance standard is established，and the algorithm for analyzing the variation of stray capacitance is proposed by conformal mapping. It is found that the air gap between the center electrode and the guard electrode is the main influencing factor of stray capacitance when both the electrode thickness and the distance between the electrode plates are determined. Combined with the finite element analysis software Ansoft Maxwell to verify the optimal air gap between the main electrode and the guard electrode, and the small capacitance standards will be realized. It has been tested that in the programmable fused silica capacitor, the actual small capacitor capacitance value of the subpF level is basically the same as the design small capacitor capacitance value.