Based on the Kolmogorov and non-Kolmogorov turbulence models, the scattering effect of atmospheric turbulence on orbital angular momentum (OAM) is analyzed and the probabilities of different OAM modes are obtained at the detecting end. The key generation rates and the maximum propagation distances of OAM-encoded measurement-device-independent quantum key distributions (MDI-QKD) under two atmospheric turbulence conditions are analyzed. The simulation results show that, with the increase of the radial intensity during the light beams are propagating in the atmospheric signal channels, the scattering effect of the turbulence on OAM is gradually enhanced, and the initial OAM states are gradually diverted to the adjacent modes with a tendency of random distribution. The probabilities of initial OAM states at the detecting end decrease gradually. The maximum propagation distance for the OAM-encoded MDI-QKD is about 10 km longer than that for the polarization-encoded MDI-QKD under the atmospheric turbulence.