Abstract: A 300MW boiler had some problems in actual operation, including the high carbon content of fly ash and furnace slag, the combustion instability and the high thermal deviation of superheated steam and reheat steam. The Realization k-ε turbulence model was used to simulate the cold state of gas-solid two-phase flow in the boiler before and after modification. The results showed that: the momentum of secondary air at the bottom increased, and pulverized coal into the dry bottom hopper dropped by 42%, from 0.077kg/m3 to 0.045kg/m3. The residence time of tertiary air pulverized coal particles increased from 30.59s to 39.97s, and the residence time increased by about 9.4s, which was beneficial to the complete combustion of pulverized coal particles. The centralized arrangement of primary air was beneficial to the ignition and stable combustion of pulverized coal. The downdip of angle of tertiary air reduced the rotational momentum of tertiary air and the increase of angle of over fire air increased the rotational momentum of over fire air, which could effectively reduce residual rotation. By putting the transformation measures into practice, the boiler could combust stably at 150MW, the combustible matter content of fly ash and slag reduced to a manageable range, the steam thermal deviation was significantly reduced, and operating parameters of the boiler was stable.