In order to solve the problem that the collected spectrum was influenced seriously by the environment light, optical modulation was used to improve the signal to noise ratio of spectroscopy-tongue inspection for the first time. First, reference light was modulated by reticle in this system; second, the reflect spectrum of the same measured object was collected under the condition that it was influenced by green, purple, red laser and environment light respectively; thirdly, the DFT was used to demodulate the collected spectrum data; the maximum harmonic components were extracted and normalized, then the spectral curve was drawn; by comparison, the spectral curves of the same measured object under different condition were almost the same. The experiment result showed that the optical modulation can remove the influence of the environment light; and this method can provide a new way to improve the signal to noise ratio of spectroscopy for tongue inspection.