The chilled mirror precision dew point meter is a device for measuring the dew point temperature of gas, and it plays an important role in the humidity measurement system. However, the important parameter for measuring the dew point, the photoelectric signal, is insensitive to the change of the mirror temperature of the chilled mirror precision dew point meter, which makes it difficult to determine the optimal voltage change value at different dew point temperatures, resulting in low accuracy of the dew point temperature finally measured. In order to determine the optimal voltage change value at different dew point temperatures, solve the problem that the photoelectric signal is insensitive to the change of mirror temperature, and find the mirror state closest to the dew point moment, a mirror image feature analysis test based on the growth law of mirror condensation was designed. The test studies the relationship between the voltage change value of the photoelectric signal and different dew point temperatures, and analyzes the law under different dew point temperatures. At the same time, an electron microscope was used to analyze the mirror image at different voltage change values. The image features were extracted by calculating the number and density of condensates. The growth law of mirror condensate at the same dew point temperature and different voltage change values was summarized, and the selection range of the optimal voltage change value for different dew point temperatures was clarified. The problem that the photoelectric signal of the chilled mirror precision dew point meter is insensitive to the mirror temperature change is solved, and the accuracy of dew point measurement of the chilled mirror precision dew point meter is improved.