The basic structure of a single-axis MEMS thermal expansion flow gyro has been proposed, and its sensitive mechanism has been revealed in this paper. The three-dimensional model of the gyro was established by the finite element method using COMSOL Multiphysics, and the changes of temperature field and isotherm of the sensitive element of the gyro were calculated with and without angular velocity. The calculation results show that the single-axis MEMS thermal expansion flow gyro has the gyroscopic effect, and the structural sensitivity of the gyro is 0.053 9 K/［(°)·s-1］with a nonlinearity of 14.13% for the input angular velocity in the range of ［-1 080 (°)/s,1 080 (°)/s］.