Method of plasma potential and electrons mean energy measurement by single Langmuir probe in highly fluctuated plasma of magnetic open trap is described. Diagnostics based on this method allows for two types of measurements: plasma potential in the emission probe mode, as well as current-voltage characteristics of the probe, followed by calculating the plasma potential and the average electron energy. All diagnostic elements, including pulse shape recorders, are fully galvanically isolated. Power supply is provided by battery placed on board of the measuring system. Control and synchronization are realized by means of optical communication lines. This diagnostic design allows measurements relate to plasma electrodes, the potential of which fluctuates with a frequency of up to hundreds of kilohertz.