The invention discloses a process system and method for producing polyoxymethylene dialkyl ethers (RO(CH₂O)_nR, n=1-8) through the continuous acetalation reaction of formaldehyde and aliphatic alcohol catalyzed by an acid ionic liquid. The process system includes an acetalation reaction unit, a product separation unit and a catalyst regeneration unit. The invention employs the separation manner combining the extraction and the rectification, which effectively achieves the separation of the recyclable materials and the catalyst where the recovery rate of the catalyst achieves more than 99%; achieves the separation of the byproduct water from the reaction system by destroying the azeotrope of water, alcohol, aldehyde, and RO(CH₂O)_nR, where the product separation is easy; the process of the catalyst regeneration is easy, and thus the catalytic cycle is achieved.