To research the influence of the defocusing of CCD on the measurement accuracy when using the joint transform correlator (JTC) to measure the image motion of space camera, the quadratic phase factor induced by defocusing is deduced based on the theory of Fresnel diffraction. Then the measurement error caused by defocusing is studied by emulation and experiment respectively for the JTC which is processed by using the method of power spectrum subtraction and binarization with threshold of zero. The results show that, the measurement error is not more than 10% when the defocusing amount is within 5% of the focal length of the Fourier lens, and the robustness is good; if the defocusing amount is increasing, the measurement error will nonlinearly increase observably. The results of the research can supply provide useful reference for the real-time and online measurement of image motion.