Disclosed is a method for detecting a combustion state of a boiler, and more particularly to a boiler and a method capable of exactly detecting an abnormal combustion state of a gas boiler using an air pressure sensor and a flame detection unit, thereby improving efficiency of the boiler. The method comprises the steps of (S1) supplying air to a burner through a fan, (S2) detecting whether an optimum amount of air is supplied through step (S1) by using the air pressure sensor, (S3) continuously detecting a status of a flame through the flame detection unit if the flame is made by an ignition part, (S4) converting a size of the flame into a corresponding voltage value, inputting the voltage value into a microcomputer, and comparing the voltage value with a preset target voltage value, (S5) stopping a combustion process if the microcomputer determines that the air pressure sensor erroneously operates based on a fact that a difference value between the voltage value and the preset target voltage value exceeds a reference value, and (S6) displaying an error message indicating an abnormal combustion state if the microcomputer determines that the air pressure sensor erroneously operates.