The invention relates to a preparation method of high viscosity monoester oil. 0.2 to 0.4 mass percent of rare earth multiphase acid catalyst is dispersed into iso-tridecanol which is 1.5 to 3 times of the weight of erucic acid, and then the iso-tridecanol is added into the erucic acid; the reactor is sealed and nitrogen gas is filled; the reactor is heated to 180 to 240 DEG C, through continuous agitation, the reaction is finished after 40 to 60 min; after the reaction solution is cooled down, the reaction solution is neutralized with sodium carbonate; then the reaction solution is filtered, the iso-tridecanol steam with water is separated and recovered (after the steam is condensed, the iso-tridecanol and the water are separated, and the iso-tridecanol is returned to the reactor), and raw ester is obtained; and the iso-tridecanol erucic acid ester is obtained through the evaporation and purification of the raw ester. The preparation method has the advantages that the iso-tridecanol erucic acid ester prepared through the invention has high relative molecular weight and higher viscosity, the other properties are corresponded to other fatty acid monoester under C 18, and the iso-tridecanol erucic acid ester can be used for preparing high viscosity lubricating oil.