A method of determining the detection phase of frequency modulation spectroscopy based on the residual amplitude modulation is proposed. According to the theoretical analysis, when there is a residual amplitude modulation in the frequency modulation spectroscopy, the signal amplitude without absorption is proportional to the cosine of detection phase. Based on this result, the detection phase could be determined directly by experiment. In order to increase the measure accuracy, a low frequency voltage control is applied to the electro-optical modulator. Comparing with the detection phase given by fitting the lineshape of frequency modulation spectroscopy with absorption, the new measured detection phase shows a deviation of less than 0.04 rad (2°), which verifies the validity of the new detection method and raises the measure speed of detection phase.