The utility model relates to a transient state three -dimensional magnetic field detecting system belongs to the power electronic electromagnetic compatibility field. Including magnetic field signal detection module, signal conditioning module, AD conversion?module, the set / module that resets, power module, the utility model discloses the magnetic field that can be used to complex electromagnetic environment such as transformer substation, electric automobile is detected, has to measure characteristics instantaneous, three -dimensional, weak magnetic field, and surveys the frequency band width, and small, convenient to use adopts the constant current source power supply to can provide stable power input to a certain extent, make the sensor be suitable for the measurement low -intensity magnetic field.