In this paper, the double heterojunction structure of GaN based blue light emitting diodes is studied by capacitance voltage method (C-V method). The C-V curve of the measured samples is divided into two parts from the change center, and the PN junction types are analyzed respectively. Then, the temperature of the diodes is controlled by liquid nitrogen, the C-V curves at different temperatures and the corresponding impurity concentration distribution curves are obtained, and the influence of temperature on them is analyzed. Finally, the double heterostructure of GaN based blue light emitting diodes is analyzed from the impurity concentration distribution curves at different temperatures. The experimental results have showed that the PN junction type of GaN based double heterojunction blue light diode is abrupt junction on both sides; At different temperatures, the capacitance of the sample increases with the increase of temperature and decreased with the decrease of temperature. The double heterostructure is gradually significant with the decrease of temperature. When T= 98 K, the double heterostructure tends to be ideal.