Abstract: To study the effect of ambient temperature on the maximum surface temperature test of locomotive lamp, changing the ambient temperature of the test through the high and low temperature humidity and heat test box, the different results of maximum surface temperature tests at seven groups of ambient temperatures were obtained. It is shown by analysis of the test results that: the ambient temperature affects the maximum surface temperature test results and produces errors. The maximum error is 17.8 ℃, which occurs at the LED lamp. Finally, it is suggested that the maximum surface temperature test of LED locomotive lamp should be carried out at the lower limit of ambient temperature.