The invention discloses a boiler combustion adjusting method for preventing high-temperature corrosion during biomass fuel mixed combustion. According to the characteristic that yellow straw and grey straw fuel are combusted in a mixed mode, a feeding system and arch heads of front and back arch water walls of a biomass direct-fired boiler are modified, the material thickness is controlled, the spreading distance is adjusted, and the flame central height is adjusted. By means of the adjusting method, high-temperature corrosion, caused by high temperature, of a heated hearth surface is avoided, stable combustion of grey straw mixed with yellow straw of the biomass direct-fired boiler is guaranteed, the method is easy to implement, and combustion efficiency is high.