A new method to measure the carrier concentration of p-GaN is proposed. The main idea is as follows: the difference between p-n+ structure GaN ultraviolet photodetector’s quantum efficiency at two different wavelengths varies remarkably with increasing reversed bias, and the most characteristic change occurs at a reversed voltage under which the p-GaN layer starts to be completely depleted, consequently the carrier concentration of p-GaN can be derived basing on this effect. The simulation results prove the validity of the idea even under the condition of high surface recombination velocity and bad ohmic contact. The thickness choice of p-GaN samples during the carrier concentration test experiment using this method is investigated. It is shown that the optimized thickness of p-GaN decreases with the increase of carrier concentration of p-GaN samples.