Abstract: Unmanned Aerial Vehicle(UAV) can obtain energy from the thermal updraft in the natural environment by autonomous soaring to improve its endurance. Mode switch is the key of UAV autonomous soaring. For the mode switch problem in autonomous soaring of UAV, an airflow sensing system was designed based on the seven-hole probe and the embedded technology. The airflow sensing system could measure the direction and speed of airflow up to 72° flow angle. Based on the airflow sensing system, the mode switch logic was designed to control the UAV to enter and leave the soaring mode. Using the wind tunnel virtual flight experiment technology, the situation of UAV encountering thermal updraft was simulated in the wind tunnel to verify the feasibility of the designed mode switch logic. Results of the wind tunnel virtual flight experiment show that under the influence of different sizes of updraft, the mode switch logic designed based on the airflow sensing system could make the UAV enter and leave the soaring mode independently, and the mode switch logic could make the UAV leave the soaring mode under different roll angle commands.