Previous studies used fixed wavelength modulated light to demonstrate the relationship between critical flicker frequency threshold (CFFT) and the factors of the subjects. We address a method to assess the whole human visual spectral sensitivity curve (with the visible range of wavelength) to CFFT by frequency modulated light. The color sensitivity curve of three cones of human eyes is discussed as well as the hardware design of the assessment system is provided. The result shows that this system could be successfully used to experiment the relationship between the stimulus wavelength and the CFFT.