A system and a method for plasma enhanced thin film deposition are disclosed, in which the system comprises a plasma enhanced thin film deposition apparatus and a plasma process monitoring device. The plasma enhanced thin film deposition apparatus receives pulsed power and a reactive gas, whereby plasma discharging occurs to ionize the reactive gas into a plurality of radicals for thin film deposition. The plasma process monitoring device comprises an optical emission spectroscopy (OES) and a pulsed plasma modulation device, in which the OES detects spectrum intensities of the radicals and the pulsed plasma modulation device calculates a spectrum intensity ratio of the radicals so as to modulate the plasma duty time of pulsed power, thereby high deposition rate as well as real-time monitoring on thin film deposition quality can be achieved.