Abstract: Based on the finite volume method, numerical simulation of three dimension MEMS micro thermal sensor is proposed using the computational hydrokinetics commercial software fluent. In the simulation, time-dependent N-S equation, realizable k-ε viscous model and no-slip wall boundary conditions were adopted. The influence of insulation layer material, gas working media and structure geometric dimension on the sensor sensitivity was studied and the measures to improve sensor sensitivity were investigated. Moreover the design of the micro thermal sensor with high sensitivity was finished. Simulation results show that the heat loss on the substrate is the smallest when the sensor is of good structure material and working media. The results also indicate that the optimized sensor geometric dimension is beneficial to the sensitivity.