Abstract: We present new progress in the diffraction grating interferometer being pre-aligned used a double frequency grating.To measure the parallelism of the double frequency grating to a nicety before built in the interferometer,a device based on Diffraction Technique for measuring the parallelism of the double frequency grating is designed.It is built of a semiconductor laser,a collimator,gratings,a precision turnplate,a beeline workbench,a redressal shelves,a ccd detector.The system error of the device is analyzed in this paper,and the parallelism of the double frequency grating is measured by this device.The results demonstrate that the diffraction measuring device suits the parallelism measured the diffraction grating interferometer based on the double frequency grating of that parallelism can attain a high pre-aligning precision.