The invention provides a circuit breaker arcing moment discriminating method and system based on a volt-ampere characteristic. The method comprises the steps that according to fault recording data, fault voltage on both ends of a circuit breaker and fault current flowing through the circuit breaker from the begin of a fault and the removal of the fault are acquired; the root mean square RMS of theacquired fault current is calculated; the root mean square RMS and a set threshold value TRMS are compared to determine whether the circuit breaker has a zero-break phenomenon from the begin of the fault and the removal of the fault, and the semi-cycle of an arcing moment is determined; and based on the arc volt-ampere characteristic, the semi-cycle of the arcing moment is processed to determinethe arcing moment. According to the invention, the root mean square method is used to judge whether the circuit breaker has a zero-break phenomenon from the begin of the fault and the removal of the fault; the semi-cycle of the arcing moment is determined; the negative resistance characteristic value of each sampling point of the semi-cycle is calculated based on the arc volt-ampere characteristic; and the arcing moment is determined according to whether the negative resistance characteristic value is positive and negative.