Aiming at the problems of high computational complexity and easy overlap of adjacent constellation points in multi-input multi-output visible light communication systems, a superposed probabilistically shaped (PS) 20-level quadrature amplitude modulation(20QAM) constellation scheme based on nonlinear coding is proposed. Firstly, based on the proposed nonlinear coding equation, a 4-level signal with equal probability is encoded into a PS 5-level signal. Then, the PS 5-level signal is mapped into a trapezoid-shaped PS 5QAM signal to maximize the Euclidean distance of the superposed constellation. Finally, the PS 5QAM signal is superposed with an equal-probability 4QAM signal to obtain a PS 20QAM signal. The experimental results prove that the proposed superposed 20QAM constellation scheme has strong robustness to nonlinear distortion of the system and better bit error rate performance than superposed 16QAM and superposed 24QAM constellation schemes.