An optical signal performance monitoring apparatus in a multi-channel optical transmission system and a method for monitoring the optical signal performance. The optical signal performance monitoring apparatus in the multi-channel optical transmission system includes an optical input unit for controlling the spot size of an inputted multi-channel optical signal and generating a 1st multi-channel beam; a diffraction and reflection unit for diffracting and reflecting the 1st multi-channel beam; and generating a 2nd multi-channel beam which is divided by wavelength and is in parallel with the 1st multi-channel beam; an optical collimation and concentration unit for collimating the 1st multi-channel beam and concentrating the 2nd multi-channel beam which is divided by wavelength; and an optical detection unit for measuring the intensity of the 2nd multi-channel beam focused by wavelength, and measuring the optical signal-to-noise ratio by measuring the optical intensity corresponding to each wavelength and an amplified spontaneous emission (ASE) noise strength at a point between optical signals.