A digital-grating-based alignment technique is brought forward and researched for digital micromirror device (DMD) maskless lithography system. Infinitesimal displacement of a silicon chip is amplified and displayed in moiré fringes generated by digital grating and physical grating. A digital-grating-based alignment model is created in DMD-based maskless lithography system. The alignment marks as well as detailed realization program is designed. Numerical simulation and preliminary experimental test is carried out. Compared to the traditional real mask, the grating digital gating characterized with variable frequency, clear image, good periodic structure and zero mask cost will extend the measurement range and reduce the displacement measurement error. This technique can realize deep sub-micron alignment accuracy, and satisfy the requirements of maskless lithography.