In the photoelectric effect experiment using a mercury lamp as the light source, in order to find the optimal preheating duration, more than twenty photoelectric effect experiment apparatuses from the same manufacturer were used, at the ambient temperature, the preheating times were recorded with a stopwatch, and the temperatures of the mercury lamp sources corresponding to different preheating times were measured with a laser infrared thermometer. Subsequently, The cut-off voltages corresponding to five types of color filters were measured by the zero current method. Furthermore, by using two air conditioners and an electric fan to lower the surface temperature of the mercury lamp, compared with the ambient temperature, under the same preheating duration, the Planck constant and the relative error were calculated. The optimal preheating time of the mercury lamp light source by the zero current method for measuring the Planck constant experiment was found. The qualitative relationship between the temperature of the mercury lamp and the measurement result of the Planck constant was analyzed.