Varied-line-space grating is very useful in synchrotron radiation devices and laser fusion devices, but there are no good ways to measure them. The line density of varied-line-space gratings have been measured by using interferometry. The principle and geometry are introduced, the methods of data processing and the results are given. On the surface of grating to be tested, quantity and density of diffracted light interferometric fringes are difference of interferometric fringes of incident light and grating groove quantity multiplied. Two sets of waves are in the common path, it is easier to make adjustment and measurement. Interference in interferometric image is removed by mid-value filtering method. Two projects are put forward for different interference fringes. The invariance of relative density is brought forward. The grating diffracts two coherent incident waves, the fringes appear, the density relates to groove density, if the incidence angles and the diffraction orders and the position of fringes are known, the groove density can be calculated. The method can satisfy the requirements of varied-line-space gratings, and its is suitable in the measurement of varied-line-space gratings and can reach some precision. Finally, the method is suggested into fabrication of holographic varied-line-space gratings.