The invention belongs to the technical field of photovoltaic application, and particularly discloses a method used for installing a photovoltaic array on an east-facing slope or on a west-facing slope. The method comprises the steps of (1) setting the oblique plane of the slope as A and determining the slope angle Alpha; (2) determining the optimal installation tilt angle Theta of the photovoltaic array in the area, determining the installation azimuth angle Zeta of the photovoltaic array, setting the oblique plane of the photovoltaic array as B and enabling the oblique plane B of the photovoltaic array to intersect with the oblique plane A of the slope on a straight line c; (3) arranging the photovoltaic array in the direction parallel to the straight line c, enabling the included angle between the straight line c where the oblique plane A of the slope intersects with the oblique plane B of the photovoltaic array and the bottom edge straight line d of the slope to be Beta, and enabling the included angle between the oblique plane B of the photovoltaic array and the oblique plane A of the slope to be Theta'. According to the method, the photovoltaic array can be installed on the east-facing slope or on the west-facing slope in the north-south direction by means of a front support and a rear support which are uniform in length specification, a support system can be manufactured conveniently, materials can be saved, installation of the photovoltaic array can be achieved more easily, construction accuracy, efficiency and quality are improved, influence caused by shading of shadows between a front row and a back row of the photovoltaic array is reduced, and then the land utilization rate and electric energy production in a power station are improved.