When outer voltage loop is closed, the stability of valley-current-mode (VCM) controlled switching converters is not only affected by variation of duty-cycle, but also affected by variation of feedback gain of compensation loop and equivalent series resistance (ESR) of output capacitor. For VCM controlled boost converter, when proportional-integral (PI) compensator is used in its outer voltage loop, its piecewise linear model is established. Based on the model, the stability mechanism of feedback gain of PI compensator and ESR of output capacitor in VCM controlled boost converter with PI compensator is studied by bifurcation diagrams. The results indicated that the stability of the converter can be reduced with increase of feedback gain, and can be improved with increase of output capacitor ESR. The validity of theoretical analyses are verified by circuit simulation results and hardware experimental results。