The invention relates to a 
coal fired boiler capable of controlling 
flue gas 
oxygen content, which comprises a furnace body provided with a fire grate and a 
hearth, wherein the lower part of the fire grate is provided with a furnace bottom wind box; 
fuel gas in the 
hearth is exhausted from a 
chimney by a draught fan after passing through a venturi passage and a dust remover; an 
oxygen content analysis sensor is arranged between the 
hearth and the venturi passage and is connected with an 
oxygen content PLC; a 
fuel gas return 
pipe provided with a circulating fan is arranged between the draught fan and the 
chimney; the other end of the 
oxygen content analysis sensor is connected with an upper layer 
fuel gas return 
pipe with an upper layer fan and a wind box fuel gas return 
pipe respectively; the upper layer fuel gas return pipe is provided with an upper layer wind control valve, is controlled by the 
oxygen content PLC and is communicated with the hearth; the wind box fuel gas return pipe is provided with a wind box control valve and is controlled by a wind box temperature PLC; and the circulating fan and the upper layer fan are driven by an 
alternating current variable frequency speed control motor and are controlled by the wind box temperature PLC and the 
oxygen content PLC respectively. The 
coal fire boiler has the advantages that: firstly, the 
coal fire boiler can run under lower excess air and the hearth temperature is low during running so that the benefit of the boiler can be improved remarkably and the environmental 
pollution caused by the emission of 
NOx is reduced; and secondly, the 
coal fired boiler can adopt more extensive coal with low ash 
melting point to enlarge the serviceable range of the boiler and prolong the service life of the 
coal fired boiler.