Abstract: A new ion acceleration method called phase-stable acceleration is proposed by use of circularly-polarized laser pulses. When the initial target density n0 and thickness D satisfy aL～(n0/nc)D/λL and D＞1 (where aL, λL, ls, and nc are the normalized laser amplitude, the laser wavelength in vacuum, the plasma skin depth, and the critical density of the incident laser pulse, respectively), a quasi-equilibrium for electrons is established due to the light pressure and the electrostatic field built up at the interacting front of the laser pulse. The ions located within the skin-depth of the laser pulse are synchronously accelerated and bunched by the electrostatic field, and thereby a monoenergetic and high intensity proton beam can be generated.