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The morphology with the interpenetrating network and optimized vertical phase separation plays a key role in determining the charge transport and collection in polymer:nonfullerene small molecular acceptors (SMAs) solar cells. However, the crystallization of polymer and SMAs usually occurs simultaneously during film‐forming, thus interfering with the crystallization process of each other, leading to amorphous film with undesirable lateral and vertical phase separation. The poly(3‐hexylthiophene) (P3HT):O‐IDTBR blend is selected as a model system, and controlling film‐forming kinetics to solve these problems is proposed. Herein, a cosolvent 1,2,4‐triclorobenzene (TCB) with selective solubility and a high boiling point is added to the solution, leading to prior crystallization of P3HT and extended film‐forming duration. As a result, the crystallinity of both components is enhanced significantly. Meanwhile, the prior crystallization of P3HT induces solid–liquid phase separation, hence rationalizing the formation of the nano‐interpenetrating network. Moreover, the surface energy drives O‐IDTBR to enrich near the cathode and P3HT to migrate to the anode. Consequently, a highly crystalline nano‐interpenetrating network with proper vertical phase separation is obtained. The optimal morphology improves charge transport and suppresses bimolecular recombination, boosting the power conversion efficiency from 4.45% to 7.18%, which is the highest performance in P3HT‐based binary nonfullerene solar cells. [ABSTRACT FROM AUTHOR]