The invention provides a method for measuring an absolute constant of a pyramid prism, which comprises the following steps: arranging a laser distance measuring total station and a measuring base in advance; fixedly arranging a plane reflector on the measuring base; respectively measuring the distances from the laser distance measuring total station to a first reflecting surface and a second reflecting surface of the plane reflector; then taking out the plane reflector; mounting the pyramid prism to be measured on the measuring base; and after measuring the distance from the laser distance measuring total station to the pyramid prism to be measured, calculating the absolute constant of the pyramid prism to be measured according to the measurement result. According to the invention, the plane reflector is used for reflecting a distance measuring laser, which does not need to enter the inside of the reflector, so that the light path is shortened, and the distance measuring laser does notneed to be transmitted in the reflector, so that the measuring precision is improved. Compared with the traditional three-section method, the method of the invention measures the absolute constant ofthe pyramid prism instead of the system constant of the total station and the prism. The invention has low requirements on a measuring field and measuring equipment, is easy to implement and has highreliability and measuring precision.