GaN-based light-emitting diode ( LED) is a typical PN junction device. In this paper，GaN-LED is taken as an example to illustrate the capacitance characteristics of PN junction device under AC small signal. We have precisely pointed out the physical mechanism contributing to the total capacitance in different forward bias voltage intervals. Under a small forward bias voltage，the total capacitance of the PN junction is mainly contributed by the depletion layer capacitance. With the increase of the voltage，the total capacitance of the PN junction is mainly contributed by the diffusion capacitance. It increases with the voltage near an e-index. These characteristics are consistent with the classic Shockley theory that PN junction capacitance consists of depletion layer capacitance and diffusion capacitance. At large voltages，the PN junction capacitance becomes negative，which is defined as negative capacitance ( NC) . After accurately analyzing the NC characteristics，the empirical formula of the NC depending on voltage and frequency is summarized. The experimental results of NC cannot be explained by the classical Shockley's theory，but it provides a strong experimental foundation for Professor Hess's“Advanced Theory of Semiconductor Devices”，which will promote the development of semiconductor device physics.