The choice of image evaluation function is a key factor for the automatic focusing of digital imaging.Aiming at the requirements of veracity and robustness,an automatic focusing evaluation function is presented based on multi-directional maximal gradient threshold method.Among the existing image evaluation functions,the gray gradient function becomes the main content due to its better real-time performance.Firstly,several typical gray gradient functions are stated in brief,and then the proposed function is deduced mathematically.At last,this function is validated by contrast tests.The results show that the proposed function has the better unimodality,unbiasedness,sensitivity and anti-interference performance,and its overall properties are obvious better than those of the traditional gradient function.