In this paper, multi-switching period oscillation phenomenon in constant on-time (COT) controlled buck converter is studied, and the effect of output capacitor equivalent series resistance (ESR) on the control performance of COT controlled buck converter is revealed. The study results indicate that ESR is the critical factor causing multi-switching period oscillation phenomenon in the COT controlled buck converter, and critical value of ESR is obtained. When the value of ESR is less than the critical value, the multi-switching period oscillation phenomenon occurs, and it disappears when the value of ESR is higher than the critical value. Finally, the theoretical results are verified by simulation.