Visible light communication(VLC) physical layer security research plays an important role in the commercialization and popularization of VLC. Based on a multi-input multiple output(MIMO) indoor VLC system, the lower limit expressions of the se-curity capacity in the case of eavesdroppers colluding and non-colluding are derived and compared. Then, an optimal beam forming scheme is proposed to maximize the security rate in the case of non-colluding. Finally, numerical simulation is carried out. The simulation results show that the confidentiality capacity in the case of non-collusion is better than that in the case of collusion, and the number of eavesdroppers only has an effect on the confidentiality capacity in the case of collusion.