Abstract: The concentration variation of byproducts(especially formic acid) with reaction time in synthesis of trioxane was studied experimentally. It was found that the formingbehavior of byproducts depends strongly upon the reaction time and the concentration of cat-alyst used,while the initial concentration of formaldehvde has little effects on it.For the re-action-distillation synthetic process of trioxane,significantly reduction in the residence timeof formaidehyde and improvement in the selectivity of trioxane were obtained by simplyadding inert organic substances with high boiling point into the reactor,which lowered therequiments for the reactor remarkably.