A light modulating system for medical treatment and health care (MTHC) is applied to sense at least one of the physiology conditions of at least one user, and accordingly modulate at least one MTHC light beam. The system comprises a physiology condition parameter sensing unit, a light output parameter calculation unit, and a light modulation unit. The physiology condition parameter sensing unit is applied to sense at least one physiology condition parameter of the user, the light output parameter calculation unit is applied to calculate at least one light output parameter according to the physiology condition parameter, and the light modulation unit is applied to modulate the MTHC light beam according to the light output parameter.