The embodiment of the invention provides an oil gas fracture main crack and net crack four-dimensional electromagnetic real-time monitoring method and system. In each fracture section construction process, a distance extroversion fracture target layer in a direction parallel to or perpendicular to the direction of a horizontal well to emit electromagnetic wave drive signals with different frequencies to obtain electric field signals or magnetic field signals prior to fracture and electric field signals or magnetic field signals in the fracture process at a plurality of monitoring points in a fracture guide well monitoring range; obtaining the residual electric field or the residual magnetic field of each monitoring point based on the electric field signals or the magnetic field signals prior to fracture and the electric field signals or the magnetic field signals in the fracture process, and obtaining an electric field residual degree or a magnetic field residual degree or an electrical resistivity residual degree according to the residual electric fields or the residual magnetic fields; and determining that the fracture crack is a main big crack or a uniform crack based on the change features of the amplitude of the electric field residual degree or a magnetic field residual degree or an electrical resistivity residual degree with the time. The problems are solved that the qualities and the features of the complex cracks cannot be effectively reflected in the prior art.