The invention relates to an underground electromagnetic flaw detector, and belongs to the field of oil and gas field engineering logging equipment. The underground electromagnetic flaw detector comprises a processor module, a transmitting system and a magnetic field array sensor, wherein the transmitting system and the magnetic field array sensor are connected with the processor module, and the processor module is in communication connection with each sensor in the magnetic field array sensor. During data acquisition, the processor module sends a synchronous acquisition instruction to each sensor to control each sensor to perform simultaneous acquisition. According to the invention, each sensor in the magnetic field array sensor synchronously receives sleeve electromagnetic response characteristics of different angles, and one-time transmitting and simultaneous receiving are achieved, so the single-time collecting period is shortened, the deviation between the recording starting angleand the recording ending angle of each sensor is reduced, the possibility of self-rotation of an instrument is reduced, the measurement accuracy is improved, the requirement for three-dimensional finedetection of a casing pipe is met, and the on-site logging efficiency is guaranteed.