In order to realize the condensation measurement of the cold mirror dew point meter with high accuracy, a photoelectric identification method based on the discrimination of two pairs of photoelectric pair tubes is proposed. One pair of the tubes is directly radiated in the measuring cavity where the cold mirror is located as the environmental control group, and the other pair of the tubes is reflected by the cold mirror surface with semiconductor refrigeration. The zero point of the third derivative curve of the photoelectric signal value is used to distinguish the time of condensation on the whole surface of the cold mirror. The experimental results show that this method can meet the needs of dew point humidity gas condensation measurement in the temperature range of - 20 ~ 25 ℃, which has technical reference significance.