A heat transfer system of water cooling closed-loop driven by the piezoelectric pump is designed in this paper. The piezoelectric pump adopts a dual-chamber series structure, and the water flow of the pump reaches 330 mL/min at the driving voltage of 150 V. In order to quantitatively analyze the influence of the vapor phase on the heat transfer performance of the water cooling closed-loop driven by the piezoelectric pump, an experimental investigation was carried out to study the influences of different filling ratio on the start-up and steady-state operating characteristics of the piezoelectric pump driven closed-loop. The experimental results show that the piezoelectric pump in the closed-loop can only be started at a fluid filling ratio of great than 0.895. When the filling ratio is great than 0.930, the presence of a small amount of vapor phase has little influence on the operating performance of the piezoelectric pump and the heat transfer system and the thermal resistance of the system is lower than 0.2 ℃/W.