The invention relates to a large-tonnage subcritical pressure steam generator. A combustor, a radiation hearth, a transition section, a lower convection section, an upper convection section and a chimney are sequentially communicated along the movement direction of hot smoke. The transition section is connected with the rear end of the radiation hearth, the lower convection section is arranged above the transition section, the upper convection section is arranged above the lower convection section, and a water-water heat exchanger with a sleeve-type snakelike reciprocation pipe structure is arranged on the outer left side of the radiation hearth and used for exchanging heat. After softened water is pressurized by a feed water pump, feed water enters a feed water inlet and sequentially passes a water passing pipeline, the water-water heat exchanger (shell side), the upper convection section, the lower convection section, the water-water heat exchanger (tube side), the radiation hearth and a tube bundle, media are heated and warmed up step by step, and high-pressure saturated wet steam is outputted. The large-tonnage subcritical pressure steam generator can technically increase the tonnage of a single-pass boiler to 40t/h, and can also increase the steam dryness to 85%.