The present invention relates to a method for preparing 1,3,5-trioxane using a reaction distillation tower including a reactor and integrally formed distillation and extraction sections. Particularly, the present invention relates to a method for preparing 1,3,5-trioxane characterized in that the amount of formaldehyde discharged to the outside of the system is reduced, to thereby increase the yield of 1,3,5-trioxane by recirculating a portion of the water phase, which is discharged through the top of the reaction extraction tower, to the extraction section, and thus to the upper portion of the extractor supply stream which supplies extractant to the extraction section.