Magnesium-based alkali is another attractive alkali source in the peroxide bleaching process of high-yield pulps. This work investigated the effluent characteristics in the high consistency retention stage and pulp properties of triploid poplar P-RC APMP (that is, so-called preconditioning followed by refining chemical treatment, alkaline peroxide mechanical pulp) by partially substituting MgO for NaOH. The results showed that the pH, dissolved and colloidal substances (DCS), chemical oxygen demand (CODCr.), cationic demand, and conductivity of the effluent in the high consistency retention stage decreased with the increase of the substitution percentage of MgO for NaOH. Meanwhile, both the light-scattering coefficient and the bulk of the pulps increased, but the tensile index and internal bond decreased. The tensile index and internal bond had a good linear relationship with bulk.