It is related to a method for preparing polyoxymethylene dimethyl ethers by a continuous acetalation reaction of trioxymethylene and methanol or methylal catalyzed by an ionic liquid. The processing apparatus used in the method includes a reaction zone, a separation zone, a catalyst regeneration zone and a product dehydration zone. A manner of circulating tubular reaction is used, resulting in a high external heat exchange efficiency, a simple structure of design and a low investment. A film evaporator is used, realizing a rapid separation and recycling of the light component, with a high separation efficiency. The separation of the catalyst solution from the crude product is simple, thereby realizing the regeneration and recycling of the catalyst.