Based on the matrix optics theory of the Total Reflection Prism Ring Laser (TRPRL) and the working theory of the ring laser in the Ring He-Ne Laser, mode-selection techniques for ring laser with the Total Reflection Prisms are studied. The clockwise and counterclockwise laser in the ring laser can work in the state of single spectral line, single transverse mode and single longitudinal mode, so as to form a stable standing wave in the ring laser, thus a stable interference fringe signal waveform is output to the photodetector. By shortening the capillary diameter of TRPRL, 3.39μm spectral lines can be suppressed; Because of the special structure of ring laser of TRPRL,the Frequency-selecting Diaphragm designed with the special split structure can be placed at any position of the optical path of the ring laser, and multiple Frequency-selecting Diaphragm can effectively suppress 1.15μm spectral line; Based on the different beam waist spot radius of laser with different wavelengths, a special elliptical small aperture diaphragm can be used at the beam waist to suppress 0.63μm Higher Order Transverse Modes of laser. It is of great significance for the application of TRPRL in Total Reflection Prism Laser Gyro(TRPLG).