MgO is a crystalline solid with significant practical and theoretical value in many fields. Brillouin scattering has long been regarded as a reliable method for accurately measuring the elasticity and photoelasticity of crystals; however, its practical application in photoelasticity has been stagnant. In this paper, three independent photoelastic constants of MgO have been measured for the first time by polarized Brillouin scattering spectroscopy and we have defined a scattering factor containing three photoelastic constants to connect with the Brillouin peak intensity. Accordingly, the ratios of |p11/p44| = 2.87 (1), |p12/p44| = 0.53 (2), and |p12/p11| = 0.18 (1) were accurately determined by comparing the intensities of transverse and longitudinal acoustic modes within the same spectrum. Then, a standard sample of CaF2 crystals was adopted to calculate the corresponding peak intensity of MgO to achieve the absolute values (|p44| = 0.085 (1), |p11| = 0.244 (4) and |p12| = 0.045 (3)). Finally, all three constants were confirmed to be negative. These first-hand Brillouin scattering results can eliminate the long-standing discrepancies of the photoelastic constants of MgO. A standard process for the simultaneous measurement of elasticity and photoelasticity through Brillouin scattering was also constructed.