The invention relates to a preparation method of a high-molecular compound, and in particular relates to a preparation method of 
microcrystalline cellulose. The method sequentially comprises the following steps: (1) in a step of acidolysis, 
plant fibers are mixed with concentrated 
hydrochloric acid in a rotary reactor in a 
mass ratio of 1:1-1.5, a mixture is subjected to acidolysis for 0.8-1.5 hours at the temperature of 70-100 DEG C to obtain an acidolysis liquid, and the 
mass fraction of the concentrated 
hydrochloric acid is more than or equal to 28 percent; (2) in a neutralizing step, 
ammonia water of which the weight is 0.01-0.05 times that of the 
plant fibers is added into the acidolysis liquid, pulping water is added until a pH value is 6-7, and raising the temperature to be between 70 and 100 DEG C to obtain a neutralizing liquid; the pulping water is 
reverse osmosis water; (3) in a filter pressing and washing step, the neutralizing liquid is pumped into a plate-and-frame 
filter press and washed by using the pulping water until 
chloride is less than 35 mg / L to obtain a wet material; and (4) in a 
drying step, the wet material is put into a flash 
distillation dryer to be dried, an air 
inlet temperature is controlled to be between 130 and 140 DEG C, an air outlet temperature is controlled to be between 70 and 80 DEG C, and then, the wet material is pulverized and screened to obtain the 
microcrystalline cellulose. The method is energy-saving, environmental-friendly, and low in production cost.