The invention discloses a system and a method for monitoring side-mode suppression ratio and channel stability of a tunable laser and relates to the device and the method for monitoring the dominant frequency and side-mode power of the laser through a sampling etalon to judge spectral quality and channel stability of the laser so as to carry out effective control. The monitoring system disclosed by the invention mainly comprises the tunable laser composed of a gain medium, a coupling optical part, a fixed lattice filter, a tunable filter and a reflective optical part, a beam collimation lens, an opto-isolator, a beam splitter component, an F-P (fabry-Perot) etalon, an optical detector, a coupling output lens, a controller, etc. Through a plurality filters outside a laser cavity and a photoelectric detection structure, wavelength lock is carried out; the spectral quality is evaluated; the tunable filter is timely adjusted and controlled; the lasing wavelength is stabilized in the required channel all the time, and good enough spectral quality is achieved. The monitoring system is suitable for online spectral quality monitoring in work of the laser and possible drift correction resulting from switching of the filter inside the cavity.