How convolutional-neural-network detects optical vortex scattering fields

•The physical mechanism of CNN fulfilling scattered OAM detection is researched.•We demonstrate that CNN converts the speckle to incident OAM TC values relies on phase front.•We demonstrate that CNN is able to learn intrinsic features of a specific disordered medium.•We demonstrate that CNN can learn the discontinuity features of the fractional OAM with phase shifts.