Water surface swimming diffuse type bensulfuron methyl herbicide for a rice field is prepared from, by weight, 10-20% of bensulfuron methyl, 20-40% of pendimethalin, 15-40% of a water surface spreading agent, 5-10% of a disintegrating agent, 5-15% of polymeric dispersant, 5-15% of binder, 5-15% of a wetting agent, 10-30% of adsorbent and 10-30% of a core material with the specific gravity smaller than that of water. A user can directly put the swimming diffuse herbicide to the water surface without going to the field during herbicide application, water soluble bags can be dissolved within 2-3 min after being put, herbicide particles are dissociated immediately, fast disintegrated in a radial shape and randomly float and expand in all the directions, the herbicide particles are continuously disintegrated from big to small, the active components are fast and uniformly dispersed to the whole water surface, a herbicide film is formed on the water surface, the whole water layer becomes suspension liquid of the herbicide, and the herbicide effect is brought into full play.