The invention discloses a method for manufacturing a multi-code-channel grating ruler based on a modulatable LCD mask. The method comprises the following steps that a mask plate is preprocessed, wherein absolute alignment marks with cross-shaped patterns and optical grating stripe patterns are formed at the positions, close to four corners, of the front side of the rectangular LCD mask plate; preparation is performed before exposure, wherein the preparation work of conducting cleaning, sputtering, prebaking and adhesive application on a whole optical grating base material before photoetching is performed; a mask pattern is modulated; single-step exposure is performed; multi-step continuous exposure is performed; the optical grating base material is diagrammed. The method is simple in process and low in cost, and the longer multi-code-channel grating ruler can be manufactured.