The focal length of measured lens can be figured out accurately according to its relation to the slope coefficient of Moiré fringes.An approach is presented to determine the location of grating centre at the level of pixel by Canny operator and detection based on mathematical morphology.The sub pixel localization is presented based on Gauss curve fitting.The localization accuracy is proved less than 0.1 pixel by calibrating the standard fringes.Then the grating is set as the reference to calibrate the equivalent length of CCD pixel to ensure the slope and width measuring of fringes accurate.The error of focal length is 0.10%.