A way is studied to obtain output laser from ring laser with total reflection prisms by prism coupler. The expressions of the reflection coefficient and the transmission coefficient of the prism coupler constructed by cuniform clearance are deduced in theory, furthermore the theoretical curve of the reflectivity and the transmittivity VS the height of the cuniform clearance showed. Problems ,such as selection of the corresponding stuff, selection of the surge ponderance in the cavity and the influence of the minishing prism couplers on the mode of the ring laser cavity are discussed. The study is considerably important for the laser gyros consist of total prisms in practical application as laser gyro and laser angle speed sensor.