The electric field of a semiinfinite parallel plate capacitor is a typical problem, which can reflect the basic characteristics for the electric field of a parallel plate capacitor. SchwarzChristopher transform is needed to solve the electric field of semiinfinite parallel plate capacitors. By using the transformation formula of ζ plane and z plane, the parametric equation of the equipotential line and the electric field line is derived, and then the explicit function equation is derived, and the curves of the equipotential line and a set of the electric field line are plotted. Besides, the implicit function of the electric field component and the magnitude and direction of the combined field strength are obtained. The threedimensional surface is drawn and the law of the electric field distribution is explained.