Abstract: In this paper, by use of the Lagrange’s equation, a vibration equation of three-particle string oscillator has been presented and its free vibration characteristics are analyzed, moreover, the modal localization phenomena of the three-particle string oscillator are studied and the "threshold value" forming its modal localization is found out. Further, by means of MAPLE 9.0 software, the curves that the amplitude ratio and the natural frequency changing with parameters are plotted. Thus some conclusions are drawn that the amplitude ratio is independent upon the parameter β and rises with the increase of parameter α; the natural frequency becomes bigger with the increase of parameter β, but smaller with the increase of parameter α; and there is a modal localization phenomena in three-particle string oscillator.