The invention discloses an optical camera system of a large-diameter geostationary orbit satellite and a testing method thereof. The optical camera system of the large-diameter geostationary orbit satellite comprises a vacuum tank, an optical camera, a camera mounting support, a turntable, a solar simulating light source, an external thermal flux simulation electric heater and a lens hood, wherein the external thermal flux simulation electric heater and the lens hood are connected with the optical camera; the optical camera is mounted on the turntable in the vacuum tank through the camera mounting support. With the adoption of the optical camera system, the accuracy of thermal equilibrium test can be improved, and the test error can be reduced; the cost is decreased.