A projection display system and a control method of controlling a projection display device capable of preventing degradation of visibility or occurrence of a sense of discomfort at a boundary between respective projection areas due to dimming are provided. In a system including a plurality of projection display devices (100A, 100B, 100C) that project image light onto a plurality of areas in one projection surface, each of the plurality of projection display devices includes a brightness detection unit (47b), and a first control unit (52) that controls a projection condition for the image light that is projected on the basis of detected brightness information, and a second control unit (53) that performs control to cause a projection condition for the image light of the second projection display device (100B) to match a projection condition determined by the first control unit (52) of the first projection display device (100A).