The invention provides a method for testing optical efficiency of a slot type solar thermal collector. The method utilizes a working platform to carry out an optical efficiency test, and includes the following steps: a slot type solar thermal collector to be detected is installed in a double-shaft rotating tracking sun subsystem; a working medium loop subsystem is connected with a thermal collecting pipe of the slot type solar thermal collector to be detected, thereby forming a test cycle working medium loop; the double-shaft rotating tracking sun subsystem adjusts the inclination angle and the azimuth angle of the slot type solar thermal collector to be detected, thereby realizing two-dimensional tracking of the sun; and an optical efficiency test is performed on the slot type solar thermal collector to be detected. Through two-dimensional solar energy tracking and temperature adjustment, the method for testing the optical efficiency of the slot type solar thermal collector can effectively avoid influence of cosine loss and radiation loss on a solar heat collection process, improves detection precision of the optical efficiency of the slot type solar thermal collector, and adopts the idea of universal design and a mechanical structure, can satisfying detection requirements of slot type solar thermal collectors of different models and sizes, and has relatively good adaptability.