The invention discloses a selection method of frequency doubling crystal optimal length of a continuous wave single frequency inner cavity frequency doubling laser. In selecting of the frequency doubling crystal optimal length of the inner cavity frequency doubling laser, influence of the frequency doubling crystal length on the stable operation characteristic of the inner cavity frequency doubling laser is taken into consideration; in selecting of the frequency doubling crystal optimal length of the continuous wave single frequency inner cavity frequency doubling laser, single frequency stable operation of the inner cavity frequency doubling laser passively realized by constantly changing the frequency doubling crystals of different lengths in experiments is not needed; the selection method has universal applicability, and is suitable for high-power, middle-power and low-power inner cavity frequency doubling laser, and the inner cavity frequency doubling laser with standard dies and other selecting elements in the cavity; and the experimental apparatus and process adopted in the invention are simple, other systems are not needed, and the cost is low.