Abstract: A phototherapy device (100) and a phototherapy instrument (500), comprising a near infrared irradiation module (110) used for emitting near infrared light to the head, and a control component (120) used for controlling the operation of the near infrared irradiation module (110) and coupled to the near infrared irradiation module (110). A method for using near infrared light for therapy, which comprises applying a first, optional second, optional third or more near infrared lights to the head of a patient. The device and method are used for treating Alzheimer's disease, improving brain mitochondrial function and ATP levels, promoting amyloid beta protein (A?) decomposition, reducing A? deposition, reducing damage to nerve cells, improving nerve tissue repair and regeneration capabilities, improving cognitive ability etc. A therapy method using the phototherapy device (100) or the phototherapy instrument (500), and a computer readable recording medium controlling required near infrared light.