The invention provides a preparation method for dialkyl dithiocarbamate and an anti-wear and rust-proof composite agent. The preparation method for the dialkyl dithiocarbamate comprises the followingsteps: step 1, performing a reaction on an alkali metal hydroxide, a secondary amine, dichloromethane and carbon disulfide, performing extraction on a reaction product through a solvent, and performing separation to obtain an organic phase; step 2, mixing the organic phase obtained in the step 1 and a water soluble acid solution; and step 3, washing a mixed liquid obtained in the step 2 by using water until the mixed liquid is neutral, and collecting the product. The dialkyl dithiocarbamate prepared by the method has excellent storage stability and compatibility.