  This letter proposes a new method based on ultrafast high energy electron
radiography to diagnose transient electromagnetic field. For the traditional
methods, large scattering from matter will increase the uncertainty of
measurement, but our method still works in that case. To verify its
feasibility, a $ 50MeV $ electron radiography beamline is designed and
optimized, and preliminary simulation of diagnosing a circular magnetic field
ranging from $ 170T*\mu m $ to $ \sim 600T*\mu m$ has been done. The simulation
results indicate that this method can achieve point-by-point measurement of
field strength. By destroying the angle symmetry of incident beams, the field
direction can also be determined. Combined with the advantages of electron
beams, ultrafast high energy electron radiography is very suitable for
transient electromagnetic field diagnosis.
