The invention discloses a method and a system for producing polyoxymethylene dimethyl ethers (DMMn) through a dual reactor. The method comprises the following steps: (1) inputting reaction raw materials into a first reaction unit, and performing initial reaction by using ionic liquid as a catalyst under a selected condition, wherein the reaction raw materials include paraformaldehyde and methylal; (2) removing the catalyst in a first reaction liquid obtained in the step (1), inputting into a second reaction unit, and reacting for a second time by using super solid acid as a catalyst under the selected condition to obtain a second reaction liquid containing DMMn; preferably separating target DMMn in the second reaction liquid through a separating unit; inputting the unreacted raw materials in the second reaction liquid or part of the second reaction liquid into the first reaction unit to react again. The system comprises the first reaction unit and the second reaction unit. By virtue of the method and the system, the low-cost, high-efficiency and environmentally-friendly continuous production of DMMn can be achieved.