For the impact of atmospheric turbulence, the improved non-constant modulus algorithm (NCMA) which is originally applied in the field of multiple-input multiple-output (MIMO) wireless communication is proposed to be applied in the OAM multiplexed communication system. This scheme presets the initial value of the equalizer based on channel characteristics, and updates the weight coefficients of the equalizer using piecewise variable step-size. By analyzing the influence of a and b in the step-size formula on the performance of the improved NCMA algorithm, the optimal values of a and b are determined. On this basis, the performance of the constant modulus algorithm (CMA), non-constant modulus algorithm(NCMA) and the improved NCMA algorithm are compared. The results show that the number of iterations of the improved NCMA algorithm is only about 1/3 and 1/5 of that of the NCMA and CMA algorithms respectively. Under weak and medium turbulence, the improved NCMA algorithm reduces the average bit error rate (ABER) by 0.7 orders of magnitude after equalization, while the NCMA and CMA algorithms only reduce it by 0.6 and 0.4 orders of magnitude. Under strong turbulence, the improved NCMA algorithm reduces the ABER by 0.3-0.5 orders of magnitude, while the NCMA and CMA algorithms only reduce it by 0.03-0.3 orders of magnitude. Therefore, the improved NCMA algorithm can effectively improve the quality of communication.