Abstract: As a ubiquitous physical phenomenon in the friction interface, triboelectrification has been widely studied due to its high sensitivity to changes in the friction interface. Among them, the triboelectric behavior of metal-polymer friction pairs has attracted much attention due to its wide application in industry and production. Especially in recent years it has been discovered that there is a close relationship between triboelectricity and friction. Not only the friction conditions affect the triboelectricity, but also the friction can be controlled by controlling the electrostatic charge on the surface. Here, we’ve designed a test system that simultaneously collects both friction signals and electrical signals. By collecting the friction coefficient, the current from the steel ball and the surface potential of polytetrafluoroethylene (PTFE) during the friction process, the triboelectric behavior of steel-PTFE was systematically studied. The fluctuation amplitude of the three signals presents a proportional relationship, which reflects the internal relationship between friction and triboelectricity. This method of collecting multiple signals at the same time is a developing trend in the field of on-line tribology monitoring in the future. In the complex friction interface, the motion of the friction pair had a great influence on the electrical signal. As for the current signal in the friction process, when the two friction pairs contacted and separated continuously, the current signal presented a bipolar signal. However, the difference of the friction mode led to the possibility that the peak shape of the current in the friction process may appear bipolar signals, or it may be unipolar. This special phenomenon was explained by analyzing the electron flow at its rubbing interface. The large interfacial potential difference of the interface caused by triboelectrification and the transfer of the third body made it possible for a large amount of static charge from the surface to re-inject to the metal surface. Because the third body participated in the friction process of the interface, the triboelectric properties of the interface were changed. The transfer of materials and the induction of the electrical polarity of the interface made the triboelectric potential showed a nonlinear trend during the friction process. That was, the surface potential of PTFE showed a reverse evolution trend after reaching the maximum value. Further experiments showed that this phenomenon of reverse evolution of surface potential existed when steel balls and steel sheets rubbed against PTFE. Optical photographs and surface element analysis of metal friction proved that there were physically adsorbed wear debris and an incompletely covered transfer film with irregular shapes on the surface of the metal friction pair. During the friction process of steel-PTFE, the reversal time of potential can be corresponded to the time when the friction coefficient entered the stable stage, which was of great value to realize the in-situ monitoring of the friction pair motion and fault warning in the process of mechanical motion. This study proposed the mechanism of the triboelectric non-monotonic change of steel-PTFE, and links the start time of stable friction coefficient with the reversal time of surface potential, which has potential application of friction monitoring of steel-PTFE or other metal-polymer friction. It is foreseeable that in the future tribology, coupling the triboelectricity of the interface or other test methods into the traditional friction test system is the trend of online monitoring of the friction state and the state of the friction pair.