The invention discloses a temperature-humidity control system for a terminal box of a substation and a control method. The temperature-humidity control system comprises a temperature-humidity controller and a heater which are arranged in the terminal box, wherein the temperature-humidity controller is electrically connected with a temperature-humidity sensor B arranged in the terminal box and a temperature-humidity sensor A arranged outside the terminal box; a control unit of the temperature-humidity controller is electrically connected with the heater; the temperature-humidity controller is connected with a computer which is connected with devices for printing, recording, communication and alarming. The method comprises the following steps: measuring temperature-humidity information inside and outside the terminal box simultaneously, randomly starting or stopping the heater, breaking the condition of generating the condensation point of air condensation of the terminal box, actively preventing condensation and carrying out control before possible condensation; and carrying out real-time temperature-humidity detection, temperature-humidity control, over-limit alarming, recording and data storage and query. The temperature-humidity control system disclosed by the invention can be combined with weather forecast, and has the advantages that the heating time can be preset, so that the condensation can be effectively prevented.