The invention relates to a large horizontal fire-tube double-flue single-reversal-chamber boiler which comprises a boiler barrel (5), two flues (2), a reversal chamber (3), a second return smoke tube (4), a front smoke box (6), a tail convection heating surface (7) and two burners (1). The two flues (2) are arranged side by side on the left and the right of the boiler barrel (5). The reversal chamber (3) is oval and disposed behind the two flues (2) and communicated with inner cavities thereof. The burners (1) are arranged side by side on the left and the right of the front end of the boiler barrel (5). The large horizontal fire-tube double-flue single-reversal-chamber boiler can not only meet limiting conditions of flue diameter and wall thickness of the reversal chamber of the fire-tube boiler but utilize internal space of the boiler barrel to the utmost extent to reduce size so that the output reaching 35t/h (29MW) is possible to the horizontal fire-tube boiler.