The flow and temperature fields caused by point heat source were obtained with the method of FEA (finite element analysis), and the pendulum characteristic of nature convection gas in two-dimensional enclosure with small size was validated and explained. The results are as follow: (1) Whether in horizontal status or in tilting status, under the function of buoyancy lift, the direction of nature convection gas always keeps vertical upwards in two-dimensional enclosure, and it has the pendulum characteristic.(2)The temperatures of two points on the same isotherm are no longer equal. When the tilt angle is 10°, the temperature difference is 20K; when the tilt angle is 20°, the temperature difference is 40K.The tilt angle can be measured by using this characteristic of MEMS airflow level posture sensor.