The invention discloses an oxygen content detection system for pyrolysis gas. The oxygen content detection system comprises a gas purification device, a condenser and an oxygen detector, wherein the gas purification device comprises a closed tank, a pyrolysis gas delivery pipe, a water delivery pipe and a spray device; a water storage area is defined in the closed tank, a blowdown port is formed in the bottom end of the closed tank, an overflow port is formed in the side wall of the closed tank, and a purified pyrolysis gas outlet is formed in the top of the closed tank; the pyrolysis gas delivery pipe extends into the closed tank from the side wall of the closed tank, and the tail end of the pyrolysis gas delivery pipe extends into the water storage area; the water delivery pipe extends into the closed tank from the side wall of the closed tank, and the tail end of the water delivery pipe extends to the position above the water storage area; the spray device is arranged at a water outlet end of the water delivery pipe; the condenser is provided with a gas inlet and a gas outlet, a condensing coil communicated with the gas inlet and the gas outlet is arranged in the condenser, andthe gas inlet is communicated with the purified pyrolysis gas outlet; the oxygen detector is connected with the gas outlet. The oxygen content detection system can detect the oxygen content of the pyrolysis gas in a pyrolysis furnace for low-rank pulverized coal continuously, efficiently and accurately.