Abstract Homogeneous porous carbons were obtained from phenolic resin/ethylene glycol resin mixtures based on polymerization-induced phase separation (PIPS) by the addition of boric acid and divalent metal iron salt. The mechanism of boric acid and divalent metal iron salt promoting pore formation was studied. The research indicates: the resulting porous carbon sample has a bigger pore size and a more uniformly distributed pore structure under the synergistic action of boric acid and a divalent metal iron salt, and its carbon matrix morphology is in the form of spherical particles. Under the synergistic effect, the average pore size of the porous carbon sample is increased to 1226 ± 48 nm. The reaction of the boric acid with the resin mixture promotes an increase in the molecular chain of the monomer, which is polymerized around the divalent metal iron salt to form a spherical polymer.