The invention discloses an online monitoring system for a power transmission line and relates to a wireless passive temperature measuring system for a high-voltage switch cabinet. The system is provided with a plurality of radio-frequency transmitting collector and sensors of which the quantity is matched with that of the radio-frequency transmitting collector, wherein a monitor is connected to the radio-frequency transmitting collector; a remote control center is connected to the monitor; the monitor comprises an analog/digital converting device, a central processing unit and a digital/analog converting device which are sequentially connected; a display unit is connected to the digital/analog converting device; an alarming device is connected to the central processing unit; and the remote control center consists of a data storage unit and a data display unit. The radio-frequency transmitting collector are used for transmitting signals and the sensors can be used for passively feeding back the signals, so that the temperature in the high-voltage switch cabinet can be measured accurately and timely and the safety is high; and in addition, the sensors in the high-voltage switch cabinet passively feed back the signals and are not influenced by an electric field and a magnetic field in the high-voltage switch cabinet, and thus measurement data are accurate. Power supply accidents due to overheat of the high-voltage switch cabinet are effectively avoided.