Traditional focusing evaluation functions suffer from poor auto-focusing stability and low sensitivity in the process of auto-focusing in UAV aerial photography due to the diversity of edge directions.In order to solve the problems，a TR focusing evaluation function based on the threshold is proposed.The function combines the advantage of the Tenengrad threshold function in reducing image noise interference and the advantage of the Roberts function in calculating gradient variations in multiple directions，and introduces a multi-directional edge gradient variation detection operator with the ability to extract gradient variations information in multiple directions.The experimental results show that，compared with the mainstream focusing evaluation functions such as SMD，Sobel，Roberts，Tenengrad，etc.，the TR focusing evaluation function has a significant advantage in two objective evaluation indexes of clarity ratio and sensitivity，with better stability and higher sensitivity，which is able to satisfy the auto-focusing needs of UAVs for accurately detecting ground targets in low-altitude environments.