The preparation of graphene by laser chemical vapor deposition(LCVD)is highly affected by the focal spot. Real-time measurement of light spot can be realized controllable preparation of graphene by LCVD method. The measuring optical system of focal facula was designed,and an improved method based on circle fitting method was presented to detect center and radius of focal spot,then focal spot area was calculated and calibrated. This method introduces some pretreatment methods,such as binaryzation,morphology processing,connectivity discrimination. Experiments show that the measured center position has average error of 5.58 pixel with true value,spot area has average error of with true value. It is better than the traditional gravity method and Hough algorithm and has faster computing speed and precision,which can realize non-contact measurement of the focus facula,and this study is of significance for quality control of other laser processing technology.