The main factor is that hair growth has to go through the anagen phase, regression phase and rest phase. Laser hair removal is 75% effective in the anagen phase, 25% in the regression phase and almost ineffective in the rest phase. Laser hair removal is based on the theory of selective thermal action. The total hair follicle sheath and hair shaft are rich in melanin vesicles, which can selectively absorb the energy of laser or photon, and then the temperature rises sharply, which leads to the destruction of the surrounding hair follicle tissue, causing the complete loss of hair root. Laser hair removal is currently the safest, fastest and permanent hair removal method. The hair is divided into three periods, namely the anagen phase, the regression phase and the resting phase. The energy emitted by the laser instrument can only destroy the hair follicles in the anagen phase, but it has no obvious effect on the hair in the regressive and resting phases, and the laser can only work after these hairs are transferred to the anagen phase, so the laser hair removal needs several treatments to be effective. If the proportion of hair in the anagen phase is small, the number of treatments should be increased. On the contrary, the number of treatments can be reduced. Based on the fact that different parts of the hair have different growth cycles, each treatment interval also differs. For example, the head hair has a relatively short resting period, so the interval can be as short as 1 month, while the trunk and limbs hair resting period is relatively long, so the treatment interval should be about 2 months. Indications Laser hair removal is suitable for all parts of dark and light colored hair but not for white hair, all parts of the body (upper and lower limbs, legs, chest, abdomen, hairline, facial beard, bikini line outside) excess black hair. Those whose hair removal by electric needle, medicine, electric tweezer, other instruments are ineffective or failed. Pain Laser hair removal is one of the less painful hair removal methods among all hair removal methods. Clinical trials have shown that most patients only feel the sensation of being “bounced by a rubber band” and that the laser hair removal instrument cools the skin at the same time.