The invention discloses a voltage measuring device and method suitable for power transmission lines. The measuring device comprises an insulator, a grading ring, a measuring circuit, a wireless transmitter or electric-optical conversion module and a voltage display and is characterized in that a to-be-measured power transmission line is supported by a fitting at the tail end of the insulator; thegrading ring is fixedly connected to the fitting at the tail end of the insulator through a connecting rod and a fixing ring in a sleeved manner; a metal sensing piece is a sheet cut off from the lateral surface of the grading ring, and the metal sensing piece is separated from the cut face of the grading ring through an insulating membrane; a sampling resistor is located inside the grading ring,one end of the sampling resistor is connected with the metal sensing piece, the other end of the sampling resistor is connected with the inner surface of the grading ring, two ends of the sampling resistor are parallelly connected to one end of a cable, and the other end of the cable is connected to the measuring circuit; the wireless transmitter or electric-optical conversion module is used for transmitting a resistor voltage signal; the voltage display is used for calculating and displaying the voltage of the power transmission line according to received resistor voltage. The voltage measuring device is small in size and low in cost.