The application relates to a system and a method for producing polyoxymethylene dimethyl ethers (DMM_n) using an aqueous formaldehyde solution as the initial reaction material. The process system of the present invention includes a polymerization reaction unit, an acetalation unit, a product separation unit and a catalyst regeneration unit. The invention uses an aqueous formaldehyde solution as an initial reaction material, the formaldehyde utilizing rate is high, and the material is cheap and easy to be obtained. Both of the polymerization and the acetalation reaction are conducted in the presence of an ionic liquid as a catalyst, which can achieve the effect that two different reactions may be catalyzed by the same ionic liquid. The present invention employs a simple catalyst system, and achieves the efficient separation of the catalyst, the byproduct water, the product and the reaction materials in a separation manner combining the extraction and the rectification.