The invention discloses a crosslinking polycarboxylic slump retaining agent and a preparation method thereof. Structure of the crosslinking polycarboxylic slump retaining agent is shown as the following formula, and the agent is formed by copolymerization of a quaternary system macromonomer, unsaturated acid, unsaturated ester and poly-propylene glycol dimethyl acrylic ester. A one-step synthesis method is adopted, and synthesis temperature is controlled to be 10-50 DEG C, so that synthesis temperature range is wide, product performance is nonsensitive to temperature, ester groups of crosslinking monomer poly-propylene glycol dimethyl acrylic ester are released slowly, slump retaining effect is good, and influence on cement hydration is small.