The invention discloses a high-precision power grid harmonic wave measurement system and a high-precision power grid harmonic wave measurement method. The high-precision power grid harmonic wave measurement system comprises a computer, a signal adaptor board and one or a plurality of harmonic wave measurement nodes, wherein the computer is connected with the signal adaptor board through a serial-port wire or a USB-to-serial wire. Power grid harmonic monitoring software which is installed in the computer can perform real-time reading of data which are transmitted from a serial port, analysis and processing, real-time item-to-item displaying, curve drawing and storage. The high-precision power grid harmonic wave measurement method comprises the steps of a first step, connecting the computer with a signal adapter board by means of the serial-port wire or the USB-to-serial wire; a second step, configuring related parameter of the harmonic wave measurement node; a third step, sampling the voltage signal and storing the voltage signal in a memory; a fourth step, reading the voltage signal from the memory specified in the third step; a fifth step, preventing execution of this step; a sixth step, storing the data; and a seventh step, repeating the steps from the third step to the sixth step. The high-precision power grid harmonic wave measurement system and the high-precision power grid harmonic wave measurement method have advantages of relatively high theoretical meaning and high practical value.