An apparatus and method for measuring a coherence sampling quality-factor (Q-factor) are provided, which are used to monitor quality of an optical signal in an optical network in real time. The quality is evaluated by a Q-factor. A laser diode and a wavelength converter are used in the apparatus to achieve wavelength coherence and amplification of the optical signal. Furthermore, the laser diode and an optical switch are used together to obtain an optical pulse that can be utilized to sample the optical signal. Therefore, after entering into an optoelectronic converter, a baseband signal in the optical signal is reconstructed through the amplification of the optical signal and the coupling of the optical pulse, so as to detect the Q-factor and to monitor the quality of the optical signal.