ABSTRACT We develop an accretion disc (AD)-fitting method, utilizing thin and slim disc models and Bayesian inference with the Markov-Chain Monte Carlo approach, testing it on the most luminous known quasar, SMSS J215728.21-360215.1, at redshift z = 4.692. With a spectral energy distribution constructed from near-infrared spectra and broad-band photometry, the AD models find a black hole mass of $\log (M_{\rm {AD}}/{\rm M}_{⊙ }) = 10.31^{+0.17}_{-0.14}$ with an anisotropy-corrected bolometric luminosity of $\log {(L_{\rm {bol}}/\rm {erg\, s^{-1}})} = 47.87 ± 0.10$ and derive an Eddington ratio of $0.29^{+0.11}_{-0.10}$ as well as a radiative efficiency of $0.09^{+0.05}_{-0.03}$. Using the near-infrared spectra, we estimate the single-epoch virial black hole mass estimate to be log (MSE/M⊙) = 10.33 ± 0.08, with a monochromatic luminosity at 3000 Å of $\log {(L(\rm {3000{Å}})/\rm {erg\, s^{-1}})} = 47.66 ± 0.01$. As an independent approach, AD fitting has the potential to complement the single-epoch virial mass method in obtaining stronger constraints on properties of massive quasar black holes across a wide range of redshifts.