The sensitivity mechanism of a thermal oscillator type biaxial MEMS angular velocity gyro was revealed. The temperature field inside the sensitive structure was calculated on the basis of the biaxial sensitivity principle, the vibration mode of the thermal oscillator and the gyroscopic effect. The results show that: A stable temperature field is formed inside the sensitive structure after 1.8 s of power-on. When there is angular velocity loading, the thermal oscillator moves with the input angular velocity, causing the temperature field to shift, and the temperature difference ΔTY(ΔTX)S between the two hot lines set symmetrically in the two orthogonal Y(X) directions shows a linear growth with the increase of the input angular velocity ax(ay), and the average temperature sensitivity of x and y axes is 121 mK/(°)/s. According to the input-output ωx-VYout and ωy-VXout characteristic curves, the mathematical model is obtained and the sensitive mechanism is revealed. The average sensitivity of x and y axes is 0.091 mV/(°)/s, the average nonlinearity is 1.86%, and the average cross-coupling is 2.3%. This paper provides a practical theoretical foundation for the optimized structure.