The invention discloses a variable-frequency automatic-control combustion water-coal-slurry and oil dual-purpose boiler with a built-in pre-combustion chamber. The variable-frequency automatic-controlcombustion water-coal-slurry and oil dual-purpose boiler is characterized by structurally comprising a boiler, a cyclone separator, a U-shaped return feeder, a processing region and a gas exhaust tube, wherein the boiler is connected through the cyclone separator through a convection flue; the U-shaped return feeder is connected to the boiler through a return flue; the processing region is connected to the cyclone separator through an outlet flue; the cyclone separator comprises the convection flue, a denitrfying agent spray nozzle and the outlet flue; the convection flue is connected to a furnace hearth; the denitrfying agent spray nozzle is connected to the convection flue through a bolt; the outlet flue is connected to the cyclone separator; and the U-shaped return feeder comprises a return flue, a return fan and a loosening fan; and the processing region comprises an over-heater, a coal saver and an ash blower. A reducing agent has sufficient reaction time with smoke gas, so thatmaterial return is avoided, and a boiler coking accident is ended, and therefore, the variable-frequency automatic-control combustion water-coal-slurry and oil dual-purpose is efficient and energy-saving.