The method of preparing fully deuterated methanol (methanol-D3) is as follows: opening the valves of deuterium gas and carbon monoxide, controlling the volume ratio of 1:1 to pre-mix first (deuterium gas and carbon monoxide are 2kg and 14kg respectively, totaling 16kg). Under the pressure of 5.5MPa, the catalyst bed of gold oxide: platinum oxide: rhodium oxide = 1:3:2 (mass ratio) was passed through 210~380C at the speed of 2L/sec. After 24 hours of continuous cyclic reaction, the fully deuterated methanol (methanol-D3) reaction solution was obtained in the separation kettle, and the obtained reaction solution, after distillation, was prepared as fully deuterated methanol (methanol-D3): 3.2kg, with the yield of 35.5%, GC purity 99.8%. Theoretical value of elemental analysis: C33.30%, D22.34%, O44.36%; measured value: C33.32%, D22.33%, O44.35%.