The projection objective of grating light modulator based on 4f signal processing system is designed, which uses LED as light source. By filtering the diffracted light via grating light modulator, the modulated image is amplified and projected. The design result shows that the total length of projection objective is 70.71 mm, and the biggest caliber is 12.40 mm. The whole aberration is controlled within an ideal range, which can satisfy the image-forming requirement. The experimental system is built based on the design result, and the experimental result is in according with theoretical analysis, which shows the feasibility of the design.