The invention relates to a four-way valve fault detecting method, a four-way valve fault detecting system and an air conditioner, wherein the method comprises the following steps: step S1, in a heating mode, acquiring the temperature T1 in the middle of an evaporator of an indoor unit, the temperature T2 at the outlet of the evaporator, an exhaust gas temperature T3 of a compressor and the temperature T4 of a middle refrigerant pipe of a four-way valve; step S2, judging whether a relationship that (T2-T1) is smaller than or equal to a or (T3-T4) is smaller than or equal to b is set up or not, wherein a is a first temperature threshold value and b is a second temperature threshold value, re-electrifying the four-way valve and returning to the step S1 if the relationship is set up, otherwise, indicating that the four-way valve has no fault, and ending the process. According to the method, conditions for judging that the reversing of the four-way valve is unsuccessful can be used for judging whether heating of an air conditioning system has problems or not more truly and accurately, so that faults are quickly found, the system is timely maintained, and the reliability of the system is guaranteed.