The invention discloses a preparation method of paliperidone (i.e. 9-hydroxy risperidone). The 6-fluorin-3-(4-piperidyl)-1, 2-benzo isoxazol and 3-(2-chloroethyl)-6, 7, 8, 9-tetralin-9-hydroxyl 2-methyl-4H-pyridino-(1, 2-a)-pyridine-4-ketone can prepare the paliperidone with high yield under the presence of the anion surface active agent in high-specific gravity alkaline brine solution. The yield of condensation reaction reaches 75%-95%, the hydrolysis side reaction of the compound 2 is obviously reduced, and the operation is simple, so that the preparation method is easy to realize the industrial production.