Abstract: An H–B–MCM–22 molecular sieve was designed for catalyzing the preparation of polymethoxydimethyl ether （PODE_n） from methylal （DMM） and polyoxymethylene. The location of the active center and the reaction mechanism were clarified through alaluminization with oxalic acid, poisoning of the outer surface and molecular dynamics. The results show that with the increase of Si/Al, the by-product methyl formate significantly decreases, and the performance is optimal when the silicon-aluminum ratio is 150. The optimized reaction conditions are as follows: time 5 hours, temperature 120 ℃, molar ratio of methylal to paraformaldehyde 1∶1. At this time, the conversion rate of DMM is 47.5%, and the selectivity of PODE_3-7 is 52.2%. Aluminum removal from oxalate and poisoning of the outer surface with dimethylquinoline will reduce the conversion rate. Meanwhile, the selectivity of PODE₂ increases and that of PODE_3-7 decreases, indicating that the acid sites on the outer surface may be the active centers of the reaction. Molecular dynamics indicate that PODE_2-7 can be generated within the pores, but due to steric hindrance, PODE_3-7 molecules have difficulty diffusing out of the pores. Based on this, it is proposed that paraformaldehyde depolymerizes into formaldehyde on the outer surface of MCM–22 molecular sieve. Formaldehyde can condense with methylal to form PODE_n on the outer surface or in the channels, but only PODE₂ can diffuse into the bulk phase within the channels.