The invention belongs to the technical field of power grid frequency detection and provides a method of detecting the frequency of a power grid based on a reduced dimension observer and a measuring device. The method of detecting the frequency of a power grid based on a reduced dimension observer comprises the steps of collecting a single-phase voltage of a power grid; building an observer and a self-adaptive law; calculating the frequency of the power grid. The measuring device comprises a voltage sensor, a signal amplifier, a filter, a frequency detector and a serial port output module. The voltage sensor collects a single-phase voltage of a power grid; the collected signals are processed via the signal amplifier and the filter and then sent to the frequency detector; via estimation of the frequency detector, the frequency of the power grid is output via the serial port output module. The research target of the method is a power grid with permanent magnet synchronous motors as driving motors; the power grid frequency detection method and the measuring device solve the problem in power grid frequency detection and improve the reliability of a frequency measurement system.