The invention discloses a relay fault diagnosis circuit and a relay fault diagnosis method. The method comprises the following steps: S01: after a circuit is electrified with low voltage, controlling a detection branch selecting unit (100) by a BMS control unit (200) to perform corresponding voltage detection branch gating operation; S02: detecting the voltage at the two ends by the currently gated voltage detection branch; S03: comparing voltage detected by the voltage detection branch and judging the adhesion state of each relay according to the comparison result; and S04: if each relay in the step S03 does not adhere to each other, performing operating control on the corresponding relay, gating the corresponding voltage detection circuit to perform voltage detection, and judging the failure state of the corresponding relay according to the detected voltage. The detection branch detection unit can perform random switching among multiple to-be-detected voltage circuits, so one voltage detection unit can detect multiple paths of voltage values, the component cost of voltage detection is greatly reduced, a BMS hardware circuit is simplified, the BMS occupied space is beneficial for being reduced and the BMS performance is more stable.