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In this paper, cascaded least mean square-least mean square (LMS-LMS) adaptive equalization scheme is proposed based on fiber coupling system, in order to improve the transmission performance of quadrature phase shift keying (QPSK) modulated optical signals in medium and weak turbulence channels. First, the transmission performance of QPSK optical signal in different atmospheric turbulence channels is simulated and analyzed according to the characteristics of optical fiber coupling system and the characteristics of atmospheric turbulence channel. Secondly, cascaded LMS-LMS equalization scheme is proposed, and its distortion signal is corrected based on the original simulation results to verify the effectiveness and robustness of LMS-LMS. Finally, atmospheric turbulence simulator is used to verify the improvement effect of cascaded LMS-LMS adaptive equalizer under different turbulence conditions. The experimental results show that when the coherence length is <inline-formula> <tex-math notation="LaTeX">$r_{0} =1.6 $ </tex-math></inline-formula> cm and the received optical power is &#x2212;34 dBm, the system can transfer the information at a rate of 32 Gbps QPSK after using cascaded LMS-LMS adaptive equalizer, and the bit-error ratio (BER) is lower than the minimum limit of forward error correction (FEC, <inline-formula> <tex-math notation="LaTeX">$3.8 \times 10 ^{-3}$ </tex-math></inline-formula>). This paper can provide theoretical guidance for the performance optimization of free space QPSK optical signal transmission system.