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, its 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 finally measured dew point temperature. In response to this issue, a mirror image feature analysis test based on the growth law of mirror condensation was designed. With the test, the relationship between the voltage change value of the photoelectric signal and different dew point temperatures was studied, and the law under different dew point temperatures was analyzed. 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 but different voltage change values was summarized, and the selection range of the optimal voltage change values for different dew point temperatures was clarified. This study solved the problem of insensitivity of the photoelectric signal of the chilled-mirror precision dew point meter to the mirror temperature change, and improved the accuracy of dew point measurement of the chilled-mirror precision dew point meter.