Optical absorption of Bi12SiO20 crystals containing nonstoichiometry defects in the cation sublattices was studied in the temperature range 80 divided by 600 K. It is shown that, at absorption coefficients the Urbach rule is fulfilled; while at indirect transitions with the participation of optical phonons take place. The value alpha(t) depends on temperature. The parameters of the Urbach rule change regularly when the Bi:Si stoichiometry is violated. The features of the isoabsorption dependences and the parameter sigma characterizing the temperature change of the edge absorption are revealed.