The invention discloses a direct-contact type waste heat recovery constant-pressure integral boiler. The direct-contact type waste heat recovery constant-pressure integral boiler mainly comprises a burner, a shell, a boiler furnace, a front smoke box, a box type heat exchanger, a rear smoke box, a flue gas waste heat recoverer and a water outlet pipe, wherein the flue gas waste heat recoverer comprises a bubbler, a floating ball electromagnetic valve, an overflow pipe, a hidden atmospheric communication opening, a sprayer and a smoke vent. The direct-contact type waste heat recovery constant-pressure integral boiler is characterized in that the shell and the flue gas waste heat recoverer are integrated, the boiler furnace, the front smoke box, the rear smoke box and the box type heat exchanger are all soaked in boiler water, and a full wet-back structure is formed. Flue gas discharged out of the boiler furnace passes through the front smoke box, the box type heat exchanger and the rear smoke box, enters the flue gas waste heat recoverer, makes direct contact with return boiler water for heat exchange, and then is discharged out of the smoke vent. The return water absorbing waste heat of flue gas enters the shell via the hidden atmospheric communication opening, and the boiler water is discharged out of the water outlet pipe on the top of the shell after being heated through the box type heat exchanger and the boiler furnace in sequence. The boiler is compact in structure, the waste heat recovery efficiency is high, smoke discharging temperature is low, and the overall heating efficiency of the boiler is high.