In this study, MgO was partially used as an alkali source in the peroxide bleaching process of bleached chemi-thermomechanical pulp (BCTMP).The effects of substitution percentage of MgO for NaOH on the bulk, optical, and physical properties of bleached pulp, and the main effluent characteristics were analyzed.In addition, the influencing mechanism of Mgbased alkali on the strength properties of the BCTMP was further investigated.Strength properties of the BCTMPs were investigated as a function of charge characteristics, fiber morphology, surface lignin content, relative bonding area, and hydrogen bonds of the BCTMP.The results showed that cationic demand (CD) and chemical oxygen demand (COD_Cr) of the bleaching effluent decreased as the substitution percentage of MgO for NaOH increased; meanwhile, the bulk and optical properties of the BCTMP increased.Nevertheless, the strength properties (tensile, tear, and burst indices) of the bleached pulp decreased as the substitution percentage of MgO for NaOH increased.The decrease in the fiber charge density and increase in the surface lignin content affected the fiber swelling, resulting in a decline in pulp interfibers bonding strength and further loss of the tensile and burst indices.