Abstract: To counter the selection and design of image focusing evaluation functions in the field of computer vision, the fundamental concept and characteristics of image focusing evaluation functions are introduced firstly. Then some kinds of current main image focusing evaluation functions (such as gray scale variation function, gradient function, image gray scale entropy function, frequency domain function) are analyzed. On the base of above discussion, a new kind of energy-spectrum entropy image focusing evaluation function is presented, and the corresponding mathematical formulae are also given. The theoretical foundation of this function is more sufficient than current other ordinary functions. Finally, the experimental results of energy-spectrum entropy function are compared with other functions, the result indicates that the energy-spectrum entropy function is more accurate, stable and reliable in the evaluation of the quantity of image focusing.