Abstract:  In order to improve the complicated pose adjustment and operation steps involved in the experiment of measuring the top angle of the prism by spectrometer，optimize the structure space of the device and improve the measurement efficiency，a double-mirror reflection method is proposed. In this method，the laser as the light source is vertically incident on the bottom surface of the prism and refracted on the slope of the prism. The refracted light is reflected several times between the two parallel mirrors and received by the target surface of the CCD detector to form a laser spot，and the center position of the spot S1 is recorded. Rotate the prism by a known small angle β，at which time the light spot shifts，record the center position of the light spot after the shift S2，and calculate the displacement ΔS between S1 and S2. Finally，the angle β，the displacement ΔS and the measurement formula are used to calculate the top Angle a. The experimental results show that when the interval of two mirrors is between 65 mm and 95 mm，and the rotation Angle β is selected between 0.1° and 0.2°，the measurement error between the measured top Angle a of the prism to be measured and the standard value is less than 0.25%. This method doesnt involve complex telescope adjustment in the spectrometer. It not only folds the system optical path，reduces instrument weight，and optimizes device space，but also has high repeated measurement accuracy by using the double mirrors and an automatic reading program interface with a CCD camera.

Key words:  the prism top angle, double mirror reflection method, angular deflection, spot displacement