The invention relates to a corner tube boiler, and particularly relates to a large-capacity longitudinal corner tube boiler. The large-capacity longitudinal corner tube boiler aims to solve the problems that an existing corner tube boiler is not supported by a steel frame, a fire grate is arranged in a single structure, the expansion direction is unidirectional when the size of the boiler is too large during operation, the sealing performance between a boiler body and the fire grate is not easily guaranteed if the expansion amount is too large, and the capacity of a transverse four-corner supporting corner tube boiler can only be limited within a small range. The large-capacity longitudinal corner tube boiler comprises the boiler body, a smoke exhaust mechanism and a tail heating surface,wherein the boiler body is connected with the tail heating surface through the smoke exhaust mechanism; and the boiler body comprises a boiler barrel, a front arch water cooling wall, a rear arch water cooling wall, a side water cooling wall, a flue water cooling wall, an outlet pipe, a steam collecting pipe, a connecting pipe, a burnout chamber, a flag type convection screen, a convection flue, aleft fire grate, a right fire grate, a supporting base assembly, an upper header, a lower header, a front left descending pipe, a front middle descending pipe, a front right descending pipe, a rear left descending pipe, a rear middle descending pipe and a rear right descending pipe. The large-capacity longitudinal corner tube boiler is used for the field of corner tube boilers.