To obtain the expression of maximum spectral luminous efficiency of mesopic level, we studied the contribution to spectral sensitivity in the mesopic region of the rods and cones based on heterochromatic brightness matching and reaction time, respectively. We next acquitted the expression of maximum spectral luminous efficiency of the mesopic level. Our opinion is that the results based on different experiments reflect different vision characteristics of human beings. The expression of maximum spectral luminous efficiency should be chosen in accordance with the application. We advise that the maximum spectral luminous efficiency based on heterochromatic brightness matching should be chosen in evaluating the effect of night scene lighting. The maximum spectral luminous efficiency based on reaction time should be chosen in designing road lighting.