Diffraction gratings are widely used in the soft X-ray and vacuum ultraviolet grating monochromators of the synchrotron radiation source, and thus the deviation of the groove density of the gratings directly affects the performance of the monochromators. In order to detect the deviation of the groove density, we built a long trace profiler (LTP) system in the pre-research process of Hefei Advanced Light Source (HALS). First, an autocollimator was used to calibrate the detection accuracy of the LTP system within 26 μrad. Then, the LTP was applied to detect the groove density uniformity of the self-developed 760 line/mm and 2400 line/mm uniform-line-spacing diffraction gratings adopted in the pre-research of HALS. Furthermore, an interferometer was employed to detect the zeroth and first-order diffraction wavefronts of the 760 line/mm grating. Finally, the above detection results were compared with the LTP-based measurement results. The results show that the root mean square error of the system calibration is 30 nrad, and the height profile curves at the same position are in good agreement in terms of LTP detection and interferometer measurement. This indicates that the LTP system can detect the groove density of diffraction gratings with high precision, providing a platform for detecting the deviation of the groove density of synchrotron radiation gratings.