Abstract Magneto-optical absorption coefficient (MOAC) in infinite square quantum well (SQW) is investigated by time-dependent perturbation method. The dependence of MOAC on the photon energy is calculated and graphically plotted. From the graphs showing this dependence, we obtained the spectral line width (the full-width at half maximum - FWHM) of the cyclotron-phonon resonance peaks using Profile method. The results show that the occurrence of peaks satisfies cyclotron-phonon resonance conditions and the FWHM of resonance peak increases with the magnetic field and decreases with the width of the quantum well. Keywords Quantum well, Infinite square potential, Magneto-optical absorption coefficient, cyclotron - phonon resonance, Full-width at half maximum.