In this paper, the capacity of a TH-PPM UWB system with an outage constraint in multi-user environments is mathematically analyzed and evaluated. To mitigate a near-far problem, a near-interference erasure scheme is adopted and the capacity of the TH-PPM UWB system using the near-interference erasure scheme is evaluated through numerical analysis and Gaussian approximation. The numerical results show that the capacity of the TH-PPM UWB system using the near-interference erasure scheme is significantly improved and there exists an optimum threshold for the near-interference erasure scheme which maximizes the capacity. The capacity which is mathematically derived is very similar to that of numerical results in the region near the optimum threshold. Furthermore, the results are more accurate as the number of bit repetitions increases.