The invention discloses a preparing method of a composite water-retention material with hydroxypropylation lobster shells being main raw materials. The method comprises the steps of firstly conducting high-temperature treatment on the lobster shells through a high-pressure steam sterilization pot, then adding the lobster shells into a diluted hydrochloric acid solution to be subjected to boiling treatment, ball-milling the lobster shells into fine powder, then dissolving the lobster shell fine powder into a sodium hydroxide solution to be subjected to freezing treatment, then unfreezing the solution, adding the solution into absolute ethyl alcohol, after adding tetramethylammonium hydroxide and propylene epoxide to conduct a reaction, adding egg-shell meal and the like to conduct an ultrasonic reaction, and then conducting treatment. The prepared composite water-retention material has an efficient water-retention and moisture-retention function, is high in adsorption and slow-release effect, can adsorb fertilizer and conduct slow release, and increase the fertilize utilization rate.