The relation between groove density error of the Rowland grating and wavelength accuracy for the Paschen-Runge mount is analyzed. An in-situ detection method for interference fringe density of the exposure field with which Rowland grating is made is proposed. A reference grating with expected groove density is put into the exposure field where the photoresist grating is exposed. The interference fringe density error of the exposure field is detected through the deviation between recording beam and its collimating diffracted light. The expression of interference fringe density error and deviation of recording beam′s collimating diffracted light is deduced. It is discovered that the interference fringe density error is related to the deviation of recording beam′s collimating diffraction light, grating curvature radius and exposure wavelength, but not the grating groove density. Taking a Rowland grating with 750.2 mm curvature radius for example, when the exposure wavelength is 441.6 nm, the interference fringe density error can be limited to 0.035 groove/mm.