Molecular orientation, with respect to donor/acceptor interface and electrodes, plays a critical role in determining the performance of all-polymer solar cells (all-PSCs) but is often difficult to rationally control. Here, an effective approach for tuning the molecular crystallinity and orientation of naphthalene diimide-bithiophene-based n-type polymers (P(NDI2HD-T2)) by controlling their number average molecular weights ($M_n$) is reported. Increasing the $M_n$ values of P(NDI2HD-T2) polymers leads to a remarkable shift of dominant lamellar crystallite textures from edge-on to face-on as well as more than a twofold increase in the crystallinity. These different packing structures in terms of the molecu-lar orientation greatly affect the charge dissociation efficiency at the donor/acceptor interface and thus the short-circuit current density of the all-PSCs. All-PSCs with PTB7-Th as electron donor and high molecular weight electron acceptor show the highest efficiency of 6.14%.