The slope of transient waveform is dominated by the characteristics of the discharging (or charging) path, including the path topology, the sizes and the states of MOS transistors. The slope value of transient waveform can be obtained by calculating the equivalent RC time constant of the evaluated cluster circuit, and it can be obtained efficiently by traversing the tree recursively. However, bottleneck effect always exists in the charging/discharging path and plays an important role on the charging/discharging behavior of the output. If neglect the effect, the waveform approximation technique used in BTS will give rise to a larger error in some cases. Therefore, we propose an algorithm to solve this problem.