A process for preparing oxymethylene polymers which comprises polymerizing trioxane or both trioxane and a specific cyclic ether in the presence of a catalytic amount of a boron fluoride-type catalyst, stopping the polymerization, and then subjecting the resulting polymer to a terminal stabilizing treatment, wherein the polymerization is stopped by adding a specific trivalent organic phosphorus compound and then the terminal stabilizing treatment is performed without removing the catalyst from the polymer.