When the traditional focus evaluation function is used to automatically focus the micro-nano structure, the diversity of the edge direction of the micro-nano structure will lead to poor focus stability and low sensitivity. In view of the above problem, the traditional focus evaluation functions are compared and analyzed,and a new focus evaluation function, i.e., Brenner2d_Roberts function is proposed by combining the Brenner function and Roberts function. Three groups of microstructure images with different edge directions are obtained through experiments, and the focus evaluation curves and related evaluation indexes are compared and analyzed. The results show that, compared with the traditional focus evaluation functions such as SMD, Brenner and Roberts, the proposed focus evaluation function shows better focus performance in multiple edge directions of microscopic images, and has better stability and higher sensitivity.