Method for adjusting contrast ratio of projector and projector

The invention provides a method for adjusting a contrast ratio of a projector. The method comprises the following steps that: an all-white image or an all-black image is input into the projector, and is projected on a projection plane by the projector to form an all-white picture or an all-black picture; a sensing unit senses the picture on the projection plane and transmits the sensed picture to a mainboard; and the mainboard controls a light source group to switch modes according to the picture on the projection plane, wherein when the projection plane is the all-white picture, the mainboard controls the light source group to be switched to a brightness mode, and a brightness value of the projection plane is a first brightness value, and when the projection plane is the all-black picture, the mainboard controls the light source group to be switched to a color mode, a brightness value of the projection plane is a fourth brightness value, and a ratio of the first brightness value to the fourth brightness valu