Optical vortex beams (VBs) possessing helical phase-front have attracted considerable attention in multiplexing communication for their orthogonal orbital angular momentum (OAM) modes. However, the mode-crosstalk and signal jitter caused by turbulence fluctuation are two main challenges in OAM multiplexing communication. Here, we introduce an intra-symbol frequency-domain averaging technology (ISFA) for turbulence mitigation. By equalizing the distorted multiplexing signals, ISFA mitigates the amplitude and phase jitter of received signals without adding system complexity and information redundancy. The experimental results show that VBs are successfully demultiplexed, and the transmission rate reaches 48 Gbit/s. After ISFA, the bit-error-rate of QPSK-OFDM signals is reduced from 1.10 × 10-3 to 6.31 × 10-4, and the error-vector-magnitude (EVM) is reduced from 31.69% to 26.29% under the turbulence strength of Cn2 = 1×10-13m-2/3 and equivalent transmission distance of 200 m. By combining ISFA with MIMO diversity gain, the EVM can be further reduced from 46.70% to 26.70%. These indicate that ISFA is available for turbulence mitigation and compatible with MIMO technology, which may have perspective potential in improving the performance of OAM multiplexing communication.