Disclosed is a method for producing crude trioxane (1) at a concentration ranging from 50 to 70 percent by weight of trioxane by trimerizing formaldehyde from an aqueous formaldehyde solution (2) in the presence of an acid catalyst (3) and concentrating, by distillation, the trioxane of the reaction mixture from the trimerization process. Said method is characterized in that the trimerization process of the formaldehyde and the concentration of the trioxane of the reaction mixture from the trimerization process are carried out in a single column (K) which is separated into a bottom column zone (A) and a top column zone (B) by means of a horizontal baffle plate (T), the bottom column zone (A) and the top column zone (B) being connected using an external steam pipe and an external liquid pipe. Furthermore, a reactive distillation process in which the formaldehyde is trimerized to obtain trioxane and the trioxane in the reaction mixture is concentrated to the solubility limit of the formaldehyde is carried out in the bottom column zone (A) at a pressure ranging from 1 to 5 bar. Additionally, a distillation process in which the trioxane is concentrated to crude trioxane having a concentration ranging from 50 to 75 percent by weight is carried out in the top column zone (B) at a pressure ranging from 200 to 900 mbar.