Reactor is a common and important equipment component in power system. In general, it can be used to limit the short circuit current in the power system and restrain the high order harmonics in the power grid. In order to determine the cause of the heating problem of series reactor in 110kV NO.3 capacitor group, after the on-the-spot investigation in the field the author puts forward three possible reasons and checks them term by term. Through the field investigation and power quality test and analysis, the author finally determines that at the time of operation, the reactor"s airway is blocked by exogenous impurity, resulting in poor heat dissipation, causing the reactor to heat. Through the engineering example, The author provides a reasonable way of researching and analyzing the heating problem of the reactor in substation. According to the method, people can quickly and accurately determine the cause of the reactor heating problem in the daily in substation and take timely measures to deal with. In addition, the author puts forward some suggestions and countermeasures to prevent the occurrence of temperature rise during the daily operation of substation reactor to ensure the safe and stable operation of the reactor.