A device and a method for positioning an inner detector in a submarine pipeline belong to the technical field of pipeline detection. Through the method provided by the invention, the position of the inner detector in the pipeline can be determined accurately according to the spread characteristic of electromagnetic waves in seawater. When the inner detector is blocked and stops moving in the submarine pipeline, an ultralow frequency electromagnetic wave emitting device mounted at one end of the inner detector can emit ultralow frequency electromagnetic waves, an ultralow frequency electromagnetic wave receiving device mounted under a sea workboat can detect ultralow frequency electromagnetic wave signals, and an upper computer can display the number values of detected electromagnetic wave signals and prompt that whether the inner detector is detected or not through voice. The invention has the advantages of high accuracy in positioning, strong anti-interference capability and strong pressure-resistant capability.