Abstract: Two kinds of Brønsted acid ionic liquids (HnmpHSO₄, HnmpPTSA) were synthesized and characterized; their catalytic activity in the synthesis of poly(oxymethylene) dimethyl ethers (DMM_n, n > 1) by the condensation of dimethoxy methane (DMM) and paraformaldehyde (PF) were also investigated. The results showed that the catalytic activity of ionic liquid was related to its acidity, HnmpHSO₄ has higher catalytic activity; the optimum reaction conditions were found as HnmpHSO₄ amount of 2.0%(mass fraction), feed mass ratio m(DMM)/m(PF) of 2.00, temperature 110℃ and reaction time 6 h, the conversion of DMM and selectivity to DMM_3~8 reached 52.28% and 49.18%, respectively. The ionic liquid HnmpHSO₄ can be naturally separated from product phase and exhibits high stability upon reusage; it still remained highly active even after being reused for five times.