In this paper, we investigate visible light communication (VLC) attocell systems in which any two neighboring attocells overlap. An efficient signal design for VLC called time superposition reuse (TSR) is proposed to mitigate interference and improve spectral efficiency. In the scheme, two neighboring cells are allocated with two time slots that have superposition in the time domain. By adjusting superposition between time slots, the system can achieve a flexible spectral efficiency and system performance. Further, we develop an optimal power allocation strategy for TSR in the system. The strategy is given according to the position of the user and the level of superposition, and then the corresponding optimal Euclidean distance is derived. In addition, we analyze the system performance and prove that the optimal Euclidean distance is an increasing function of the delay between signals and a decreasing function of the sampling period. Simulation results demonstrate that the optimal power allocation has a better performance than uniform power allocation and TSR outperforms time division multiple access significantly for the user in VLC attocell systems.