Polymeric membranes with porous structures have attracted significant attention across a wide range of research fields. Nonsolvent-induced phase separation (NIPS) generates pores during the phase separation of a polymer solution via the nonsolvent penetration. In this work, an aliphatic polyketone (PK) was employed as a polymer matrix for NIPS, and an alginate additive was doped into the solution to induce either an accelerated or inhibited solvent-nonsolvent exchange rate controlled by alginate mobility during phase separation. As a result, the pore size of PK membrane was controllable and a desirable water permeance was achieved. Furthermore, the alginate additive within the membrane matrix also improved hydrophilicity, which is favorable for water transport and oil-fouling resistance. The oil-in-water (O/W) emulsion separation performances of optimized membranes largely depended on their altered membrane pore structures and properties, indicating the effectiveness and feasibility of this approach for NIPS-derived polymeric membranes. Alginate-rendered control of phase separation may also be applicable for the fabrication of other polymeric membranes that require pore modifications.