The aim of this study is to determine the focusing position of camera relative to the measured point by monocular vision algorithm, and a new monocular vision algorithm is presented. The methods of position-from-defocus and position-from-focus in monocular vision method are combined to determine the focusing position. A new kind of energy-spectrum entropy image focus criteria function and the calculation model of position-from-defocus method and position-from-focus method are given. The verification experimental results indicate the stability and reliability of the proposed algorithm.