The invention provides a method for controlling temperature of a butyl rubber reactor. The method includes the steps that signals of a pressure transmitter of an ethylene tank are fed back to adjust a pressure regulating valve of the ethylene tank, the pressure of the ethylene tank is controlled below first pressure, the liquid column pressure difference value between normal ethylene liquid level elevation and heat exchange tube bundle top elevation of the reactor is subtracted from ethylene vapourizing pressure below minus 100 DEG C, and the obtained difference value and an adjusting margin value between the pipeline resistance drop from an outlet of the ethylene tank to an inlet of a heat exchange tube bundle of the reactor and a 10KPa adjustable decompression element are added to obtain the first pressure value; pressure drop of the adjustable decompression element is adjusted, so that the pressure of ethylene entering the heat exchange tube bundle of the reactor is equal to the ethylene vaporizing pressure below minus 100 DEG C. The method ensures that reactions are conducted stably below minus 100 DEG C, prolongs polymerization time, reduces the probability of reaction unexpected interrupt, improves productivity and production efficiency and reduces instability of the polymerization process. The method is simple in technological flow path and easy to operate, high in control accuracy, low in operation cost and safe and reliable in production, installation is convenient, and energy does not need to be consumed in the process.