The utility model discloses a combustion furnace for producing low-degree phosphoric acid by utilizing poor sludge phosphorus, comprising a furnace body, a grate (3), a raw material feeding hole (2) and a product outlet (7), wherein a hearth is arranged in the furnace body and comprises a combustion chamber (6) and an oxidizing chamber (8); a divider wall (9) is arranged between the combustion chamber (6) and the oxidizing chamber (8); the grate (3) is arranged in the combustion chamber (6); the oxidizing chamber (8) is provided with an air quantity control hole (10); and a tuyere adjusting plate (12) is arranged in the air quantity control hole (10). Compared with the prior art, the utility model improves the furnace body; hard coke (or charcoal) can be added to sludge phosphorus to carry out mixing combustion during working, thereby increasing the combustion temperature and the phosphorus combustion ratio and greatly increasing the phosphorus yield which can reach more than 95 percent.