The continuous wave electro‐optic probing (CWEOP) technique is for the first time used to probe the GaAs Czochralski liquid encapsulated semi‐insulating surface property. From the second derivative profile of the signal, the space‐charge boundary under different bias is clearly observed. The motion of the detected space‐charge boundary with applied bias shows that the substrate surface evolves from a highly compensated semi‐insulating surface to ap−surface with thermal annealing. The CWEOP experiment also concludes that there is no other level except EL2 possessing enough concentration to compensate the background shallow acceptor. Since this technique is extremely sensitive to the GaAs surface property, it can be used to directly probe the surface potential, field, and charge distribution in GaAs material and device.