For the traditional Brenner definition evaluation function, which is not ideal in the image evaluation with large amount of defocus and complex content, this paper proposes an improved Brenner definition evaluation function.On the basis of the Brenner evaluation function, this function calculates the image gradient by adding a set of vertical mask template and combining the horizontal direction template of Brenner operator itself.Finally, the effectiveness of the improved method is verified by comparing the different fuzzy degrees and the noisy images.Experimental results show that the improved method on the basis of the Brenner function improved the accuracy at the same time, the calculation time is better than other common gray evaluation function, effectively improve the traditional Brenner definition function evaluation effect.