The imaging quality of camera severely depends on the precision of location of focal plane, and correlation research on the testing method is carried out. A testing method of locating focal length based on the image spectrum is presented, which can make use of at least two defocusing pictures from the same target to calculate the defocusing amount of camera image plane. And on the condition that the camera lens is aligned well with the design requirement,the target is random. In the process of numerical calculation using the similarity of two images, the image spectrum can avoid direct decomposition and make the testing easy, stable, reliable and with high precision.