For the lightning overvoltage on-line monitoring device of the transmission line, it is very important to accurately identify the lightning current signal. At present, whether it is hardware triggering or software triggering, there are triggering problems such as false triggering and leakage triggering, and the current monitoring system has obvious disadvantages in terms of economy and convenience. This paper proposes an embedded lightning strike online monitoring system, which can realize efficient and convenient real-time monitoring of transmission lines, and greatly improve the convenience and economy of lightning strike monitoring devices. At the same time, a triggering algorithm of first detection and identification is proposed to accurately identify the lightning current transient signal. First, the lightning current signal (bad data) is quickly detected by the method of amplitude difference, then the bad data is accurately identified by the method of period difference. Through analysis experiments, it is proved that the method can reduce the number of false triggers and missed triggers of the monitoring system. And through laboratory tests, it is proved that the acquisition accuracy of the monitoring system is 98.49%, which meets the needs of real-time monitoring and provides an economical and convenient monitoring system for online monitoring of transmission lines.