A new on-line detection method was proposed to detect the opening delay time of electronically controlled gasoline injector. The method detected the opening delay time of fuel injector by analyzing the driving current waveform. Only when the armature of solenoid valve fully pulled in, the fuel injector would fully open. And the driving current would hence produce an obvious inflection point. This method constructed a hardware circuit and detection logic, which could trigger the detectable pulse when the driving current had the inflection point. By analyzing the delay time and pulse width of triggering pulse, the delay time of injector could be accurately obtained. The technology could be used for factory inspection and routine maintenance of fuel injector. It could also be transplanted to ECU to conduct online detection for the delay time of fuel injector, which provided the compensation basis for fuel injection timing strategy.