A sub-critical pressure steam injection boiler comprises a steam pipeline composed of a water pump, a water supply pipe, a steam discharging tube, a radiation tube bundle, a convection tube bundle, a steam separator, and a feed-water pre-heater. The radiation tube bundle is composed of connected a plurality of horizontal U-shaped straight tube rows, the convection tube bundle is composed of connected a plurality of U-shaped smooth tubes and a plurality of U-shaped finned tubes, the feed-water pre-heater is composed of a shell and an inner sleeve while the shell is provided with a shell side inlet and a shell side outlet, two ends of the inner sleeve are provided with a tube side inlet and a tube side outlet, the shell side inlet is communicated with the water supply pipe while the outlet is communicated with the inlet of the convection tube bundle, the outlet of the convection tube bundle is communicated with the tube side inlet, the tube side outlet is communicated with the inlet of the radiation tube bundle which outlet is communicated with a steam inlet tube of the steam separator which steam outlet tube is communicated with a steam discharging tube. The utility model has simple and compact steam pipeline, stable operation, high safety and reliability, and high heating efficiency, so the problems that high pressure block in deep wells and super-heavy oil block are hard to develop are resolved.