A water-retention and dust-suppression crop straw mulching method includes crushing a whole harvested crop straw to be 1-4 cm in length first, mixing the crushed crop straw with an adhesive well, and uniformly paving the mixed crop straw into a forming die for extrusion forming to form crop straw modules. A plurality of drain holes is on the crop straw modules. The method includes stubble cleaning and subsoiling treatment on the harvested farmland, and then performing rotary cultivation on the farmland to loosen soil, and performing rolling treatment on the rotary-cultivated soil by a land roller; paving the crop straw modules on the farmland uniformly in a densely covered way, meanwhile, connecting and fixing the crop straw modules by ribbons and connecting rods. The method helps to block the upward evaporation channel of soil moisture, reduce the evaporation of soil moisture, prevent the crushed crop straw from polluting the environment.