Flat-field correction can remove the non-uniformity in the coronagraph imaging process, which is a necessary step in its data calibration. The device and method for measuring flat-field of coronagraph based on an opal glass are proposed in this paper. Some simulations and site measurements are carried out to verify the feasibility of this method. First of all, the uniformity of the diffused sunlight passing through the opal glass in the field of view of the coronagraph is simulated. The simulation result shows that its uniformity is 99.98%, which is very close to the ideal and uniform area source. Moreover, the flat-field of the 12 cm ground-based coronagraph is measured. The observational result indicates that the non-uniformity of the coronagraph imaging can be observed with this method, such as the fringes on the detector. The result after flat-field correction agrees well with the radial distribution of the background luminance of coronagraph and sky. Lastly, the flat-fields from the opal glass are compared with the sky flats in order to evaluate the validity of flat-field measurement with opal glass. Their correlation coefficient is 99.94% and the corresponding linear fitting slope is 1, which shows a strong correlation.