The utility model relates to a formaldehyde production in -process increases device of by -product steam, including the reactor, the bottom of reactor is connected with the vaporizer through first pipeline, the output of vaporizer passes through second tube coupling shell and tube heat exchanger, the bottom of vaporizer is still through the top intercommunication of back flow with the reactor, oneside upper portion of shell and tube heat exchanger is provided with feed water outlet conduit, and the lower part sets up the feed water inlet channel, shell and tube heat exchanger's bottom is passed through export pipeline and is connected with the absorption tower, the bottom on absorption tower is passed through the output tube and is connected with the circulating pump, the delivery outletof circulating pump is connected with shell and tube heat exchanger through spraying the liquid pipeline. The utility model discloses compact structure, reasonable, convenient operation through increasing the byproduct steam device, can effectively utilize the reaction produced in the reactor heat and formaldehyde absorption heat, improves steam output, falls the heating load of low absorption tower simultaneously, reduces the cooling water quantity, the reduction energy consumption.