The invention provides a high-power inner cavity frequency doubling single-frequency laser device. The high-power inner cavity frequency doubling single-frequency laser device comprises a pumping source (1), a gain crystal (2), a ring-shaped resonant cavity, an isolator, a frequency doubling crystal (6) and a thermal lens effect compensating plate (5), wherein the isolator is composed of a magneto-optical medium (3) and a half wave plate (4) which are placed in a permanent magnet. The thermal lens effect compensating plate (5) is placed close to the magneto-optical medium (3). The thermal lens effect compensating plate (5) is an optical crystal thin plate with a certain thickness, has a negative thermo-optical coefficient and is provided with two parallel light-transmitting faces. The thermal lens effect of the magneto-optical medium (3) can be dynamically compensated for. The thermal lens effect of the magneto-optical medium is dynamically compensated for through the crystal thin plate with a certain thickness and a negative thermo-optical coefficient, the inside-cavity light beam quality degradation caused by the thermal lens effect of the magneto-optical medium can be relieved or eliminated, the stable area of the laser device is expanded, and therefore the high-power inner cavity frequency doubling single frequency laser device stably running is obtained.