The invention discloses an on-line detection device for open-circuit fault of power tubes of an inverter and a detection method thereof, relates to the field of on-line detection, and solves the problem of low detection speed existing in the conventional detection method and detection device. The detection device consists of three on-line detection circuits for the open-circuit fault of the power tubes; the circuit is used for detecting the open-circuit fault state of two power tubes in a bridge-arm circuit, and consists of a photocoupler circuit for detection, a logic circuit and a rising-edge delay circuit, wherein the rising-edge delay circuit receives a switch signal for time delay to acquire a time-delay switch signal, and sends the time-delay switch signal to the logic circuit; meanwhile, the photocoupler circuit for detection receives the switch signal controlled lower output signal, and transforms the output signal into a logic signal and sends the logic signal to the logic circuit; and logical operation is carried out to obtain the fault state, when the logical operation result is zero, the power unit correspondingly controlled by the switch signal is in a open-circuit fault state, otherwise, the power unit is normal. The invention is applicable to the open-circuit fault diagnosis of the single pipe or bridge arm of the inverter in various controlled motor drive systems and power-supply systems with current-open or close loop.