A device and a method for testing a steel defect based on internal and external magnetic perturbation. The device includes: a magnetizer comprising a magnetization source and a magnet yoke, arranged on a surface of a sample, and configured to generate two types of typical magnetic field regions applied to testing based on internal and external magnetic perturbation; a double-row magnetic sensor probe, configured to collect internal and external magnetic perturbation data; a master controller, configured to perform pre-processing on the internal and external magnetic perturbation data, and store the pre-processed data; scanner wheels, configured to generate a sampling trigger pulse during scanning to enable the master controller to receive the internal and external magnetic perturbation data from the probes; and a host computer, configured to analyze the pre-processed data uploaded by the master controller to obtain a defect quantitative result.