The invention discloses a firing method of a wall type opposed firing boiler. A plurality of burners and over-fire air spraying openings located above the multiple burners are arranged on a front wall and a back wall of the wall type opposed firing boiler respectively or are arranged on the front wall of the wall type opposed firing boiler only, and the multiple burners are arranged in a matrix mode, are arranged to be in M lines from bottom to top and are arranged to be in N columns from left to right, wherein the M is an integer larger than or equal to 2, and the N is an integer larger than or equal to 1. The firing method includes the steps of controlling the average excess air coefficient of a main firing area where the multiple burners are located to be smaller than 1, controlling the average excess air coefficient of a firing area between the over-fire air spraying openings and a hearth outlet of the boiler to be larger than 1 so as to achieve in-boiler air grading firing, and controlling the average excess air coefficient of the burners in the first line to be larger than the average excess air coefficient of the burners in the second line to the Mth line. By means of the firing method, the aim of in-time firing can be achieved.