The invention discloses a preparation method of pseudo-boehmite with high purity and high crystallization degree. The method comprises the following steps: (1) bottom water is added into a pressure zone and a reaction zone of a reactor, and a stabilizing agent is added into water in the reaction zone; (2) an aqueous solution of aluminium salts and an alkaline solution are respectively injected into a feed inlet I and a feed inlet II at the upper part of the reaction zone of the reactor, a sol forming reaction is carried out, during the sol forming reaction process, when aluminium hydroxide sol reaches a certain concentration, a pressure pump pumps water in the pressure zone into a reaction zone through a semi-permeable membrane, the sol is kept in a concentration range, and the reaction is continuously carried out for a period of time; (3) after the reaction ends, a stirring paddle at the bottom of the reactor is started, pH value of the reactor is adjusted, temperature is rapidly raised to an aging temperature, after aging, filtering, washing and drying are carried out, the pseudo-boehmite is obtained. The pseudo-boehmite is prepared by a semi-permeable membrane penetration mode, and the obtained pseudo-boehmite has the advantages of high purity, high crystallization degree, and uniform particle size distribution; and the pseudo-boehmite is suitable for preparing carriers and catalysts of hydrotreatment catalysts for wax oil, heavy oil and other inferior raw materials.