Abstract: In the Mars exploration missions, due to the low-pressure atmosphere, the heat transfer on the Mars surface is different from that in other in-orbit environment. It is necessary to test the Mars rover in a simulated low pressure and Martian atmospheric environment for the thermal model correction, the early failure screening, and the performance verification. One has to measure a wind speed of 0 to 15 m/s in the thermal test of the rover under a pressure of 1400 Pa . In this paper, the heat transfer model on the surface of a constant heat flow hot bulb probe is established by the dimensional number analysis method. The output signal and the natural convection influence under a low pressure are analyzed with the model, and the results are compared with the test results under the corresponding low pressure. It is shown that the forced convection plays a dominant role in the convection around the hot bulb probe under the pressure of 1400 Pa, and the sensitivity of the probe is about 0.1 to 0.2 mV/(m·s^-1), which is adequate to be used in measuring the wind speed under a very low pressure.