The present disclosure provides a device and method for detecting a defect in a main shaft of a wind turbine. The device includes: an excitation source, configured to generate an electromagnetic ultrasonic guided wave signal; a nickel strap, magnetized and disposed on an outer surface of an end of the main shaft; a coil, disposed at the nickel strap, configured to receive the electromagnetic ultrasonic guided wave signal such that the electromagnetic ultrasonic guided wave signal propagates in the main shaft, the coil and the nickel strap being configured to transform the electromagnetic ultrasonic guided wave signal propagating in the main shaft into an electrical signal by electromagnetic induction; a signal collector, configured to collect the electrical signal and transform the electrical signal into guided wave detection data and a wireless communication component, configured to transmit the guided wave detection data to a remote equipment.