The invention relates to the technical field of preparation of paraformaldehyde, and provides a preparation method of paraformaldehyde. The preparation method of paraformaldehyde comprises the following steps: evaporating concentrating a formaldehyde aqueous solution with a weight concentration of formaldehyde of 37 percent at the temperature of 50 to 60 DEG C until the weight concentration of theformaldehyde is 70 to 82 percent, introducing an atomizing granulating tower with the concentrated formaldehyde aqueous solution, then spraying out in a droplet shape by virtue of a spray nozzle in agranulating device, introducing nitrogen at the temperature of 30 to 60 DEG C, and condensing the droplet-shaped formaldehyde aqueous solution into granular paraformaldehyde by virtue of the nitrogen; and drying the granular paraformaldehyde by utilizing far-infrared rays until the weight content of the formaldehyde is 92 to 97 percent, and thus obtaining the finished product paraformaldehyde. Avibrating-type far-infrared dryer is used for substituting a vibrating fluidized bed dryer in the prior art to prepare the paraformaldehyde with high formaldehyde content, so that the drying time is short, the granularity distribution of the product is uniform, the energy consumption is low, and the solubility of the product is good.