The invention provides a static laser speckle reduction method combining a multimode optical waveguide and a diffractive optical device. The method comprises the following steps that Step 1, a laser beam is collimated and then input into the diffractive optical device, the diffractive optical device outputs the incident collimated laser beam as diffraction beams with different diffraction orders;Step 2, the diffraction beams with the different diffraction orders are focused by a lens, the focused beams are input into the multimode optical waveguide, the multimode optical waveguide modulates the transmission delay of different diffracted beams, so that the diffraction beams of all diffraction orders are output from the multimode optical waveguide at the delay difference delta T; Step 3, the diffraction beams output at different delays are collected by an imaging lens and imaged on a projection screen, the projection screen is provided with a rough surface, an image on the projection screen is received and processed by a digital camera, and the static laser speckle suppression is achieved. The static laser speckle reduction method combining the multimode optical waveguide and the diffractive optical device has the advantages that the reliability is high, electric energy supply is not needed, the response time is short, no mechanical running mechanism is needed, the system is simple, the size is small, and the cost is low.