Abstract: An experimental method of small semiconductor laser weak light source for measuring the Planck constant based on the photoelectric effect is adopted. It has the advantages of high precision， small volume， light weight， low cost， save electricity，convenient. It has been demonstrated that the semiconductor laser light source has a good monochromaticity compared with the related other light source by the spectral analysis. The reasons of influencing the measurement accuracy because of the monochromatic spectral line width are analyzed theoretically.

Key words: small semiconductor laser, photoelectric effect, Planck constant, monochromatic spectral , line width