For the high temperature superconducting (HTS) coils supplied by DC powers, a thermal disturbance may cause local temperature rise and irreversible damage because of constant current. In contrast, when a thermal disturbance occurs, the current flowing in a closed-loop HTS coil operated in persistent current mode (PCM) will decrease correspondingly. In this paper, a closed-loop single-pancake HTS coil energized by persistent-current switch (PCS) is tested to obtain the minimum quench energies (MQEs) and normal zone propagation velocity (NZPV), which we use to describe thermal stability, as well as an unclosed one. A FEM model coupling with PDEs and thermal module is built to reveal the mechanism. The experiment results coincide with the simulated ones and show that the closed-loop coil has larger MQEs and better thermal stability.