Abstract: In order to observe the phenomenon of the filament temperature drop caused by the board-current, the ideal diode filament temperature was detected precisely with an infrared thermometer. Since the cathode filament temperature drop caused by the boardcurrent was neglected, it resulted in about-1.86% system relative error in the measured metal work function. The phenomenon was found through a theoretical analysis, and it was verified by an experiment.When both the filament current and board-current are lower, the variation of the filament temperature with the board-current is not measurable; when the filament current is above 0.74A, it can be clearly observed that the filament temperature is (1～2)K lower than that of zero board-current after the board-current is outputted.