A low-complexity spectrum resource allocation algorithm with near-optimal system throughput is proposed to resolve the conflict between high system throughput and low complexity of the multi-cell resource allocation algorithm for indoor ultra-dense visible light communication (UD-VLC) networks. Firstly, through establishing the optimal model of the resource allocation problem in each cell, we derive the conclusion that the problem is a convex optimization problem. Then, the analytic formula of the normalized scaling factor of each terminal for resource allocation is derived after reasonable approximate treatment, and the resource allocation algorithm is proposed. Finally, the complexity analysis shows that the proposed algorithm has polynomial complexity, which is lower than the classical optimal inter-point method. The simulation results show that the proposed method achieves 57% performance improvement on average system throughput and 67% performance improvement on quality of service (QoS) satisfaction against the required data rate proportion allocation (RDR-PA) method.