The invention provides a method for preparing trans-1,3,3,3-tetrafluoropropene from a mixture through normal-temperature isomerization. The method comprises the steps of conducting a gas-phase dehydrofluorination reaction for dehydrofluorination and dehydrochlorination in a first reactor with a mixture of pentafluoropropane and tetrafluoromonochloropropane as a raw material by introducing anhydrous hydrogen fluoride; introducing all the mixture obtained in the first reactor into a second reactor and conducting a gas-phase isomerization reaction with chromic fluoride as a catalyst at normal temperature and normal pressure; and separating pentafluoropropane, tetrafluoromonochloropropane, hydrogen fluoride, hydrogen chloride and cis-1,3,3,3-tetrafluoropropene in a reaction product to obtain a purified product, namely the trans-1,3,3,3-tetrafluoropropene. The raw materials are easy to obtain. The process route is simple and flexible. The conversion rate is high. The target product is good in selectivity and low in cost. According to the method, the temperature of the isomerization reaction is reduced to the normal temperature; the chromic fluoride catalyst is prepared for the normal-temperature reaction; and moreover, the incoming material for the isomerization reaction is all the unseparated material obtained from the reaction in the last step.