The invention discloses an image focusing measure implementation method based on number theory transformation, and belongs to the technical field of passive imaging. The method comprises the followingsteps of firstly, carrying out noise reduction processing on an image, then partitioning the image after noise reduction, and calculating the number theory transformation of each sub-image to obtainthe frequency domain coefficient of the corresponding sub-image, and selecting the frequency domain coefficient of each sub-image to construct definition information of the image, and finally, solvingthe variance of a set formed by the definition information of all the sub-images, and taking the variance value as the focusing measure of the whole image. The detail information of the image is extracted by adopting the number theory transformation on the basis of carrying out noise reduction and partitioning on the image, the method has the advantages of simple principle and low calculation complexity, meanwhile, through implementation of the steps, the interference of noise on the detail information of the image is reduced, and particularly, the noise robustness under the low-contrast imaging condition is high. The method is suitable for a passive imaging system of the camera and is convenient to popularize and use.