This letter investigates the secrecy performance of a visible light communication (VLC) system with a legitimate receiver and a group of eavesdroppers. Considering the randomness of the locations of the legitimate receiver and the eavesdroppers, the statistical characteristics of the signal-to-noise ratio over the VLC downlinks are characterized; then, we derive the closed-form analytical expressions for secrecy outage probability and the average secrecy capacity by using stochastic geometry method. Finally, simulations are carried on to verify our proposed analytical models.