Ground Wide Angle Cameras (GWAC) of the astronomy satellite SVOM contains 36 wide-angle telescopes, and its high-precision automatic observation of short exposure is established on real-time auto-focusing, in this paper, we focus on the research and realization of image definition evaluation for real-time auto-focusing on GWAC. At first, principles of image definition evaluation methods of common telescope and applicability on GWAC system were researched, thus two kinds of methods based on encircled energy of star image were obtained, namely radius of 50% energy of stars and the full width of half maximum (FWHM) of point spread function (PSF), were suitable for GWAC system. Different from time-consuming algorithm of common astronomy software package IRAF, method of calculating FWHM via PSF fitting calculation based on source intensity distribution was proposed, and further influence of key method parameters such as fitting model, star-choosing standard, centring precision, fitting radius, interpolation method, imterpolation interval, FWHM after-treatment, etc on calcu-lation precision and speed of FWHM was researched. A set of definition evaluation methods that are applicable to GWAC real-time automatic focusing were acquired in the last, and it is realized by C++ programming. Calculation error of FWHM method in the paper is 0046 pixel, the precision is equal to IRAF approximately and calculated focus location is the same; calculation time of single graph (about 300 stars after selection) is 0.67 s which is 1/20 of calculation time for IRAF. It satisfies precision and real-time requirements of automatic focusing for GWAC system. The research results havebeen applied in GWAC system and provide reference for other automatic observing system of telescope.