The invention discloses a power cable middle joint insulation state online detection device and relates to power cables. Through the power cable middle joint insulation state online detection device,convenience is brought to online monitoring, and detection progress is accelerated. The device comprises an ultrasonic sensor, a Rogowski coil, a conditioning circuit A, a conditioning circuit B, an A/D conversion module, an ARM processor, a communication module and an upper computer, wherein the ultrasonic sensor is pasted to the surface of a cable joint through an epoxy resin coupling agent, andthe ultrasonic sensor converts a collected ultrasonic signal into an electric signal and transmits the electric signal to the conditioning circuit A; the Rogowski coil transmits the collected electric signal to the conditioning circuit B; and the conditioning circuit A and the conditioning circuit B are connected with the A/D conversion module respectively, and the A/D conversion module, the ARMprocessor, the communication module and the upper computer are sequentially and electrically connected. Through the device, working efficiency is improved, and therefore the situation that a huge economic loss is brought by time waste is avoided.