The invention provides a method and a device for on-line measuring the inner temperature of a cable joint, the device includes a built-in temperature measurer and an outside receiver. The built-in temperature measurer measures the temperature of the cable joint or a core wire surface by means of direct contact measuring method, and transmits the data to the outside receiver through an insulating layer by means of the radio frequency identification technique. At the same time, the outside receiver supplies power energy to the built-in temperature measurer by means of the radio frequency signal, so it resolves the problem of supplying power to the built-in temperature measurer. The device of the invention has the advantage of accurately measuring temperature, small volume, superior applicability and so on.