In traditional in situ polymerization preparation for solid‐state electrolytes, initiators are directly added to the liquid precursor. In this article, a novel cellulose paper‐based composite separator is fabricated, which employs alumina as the inorganic reinforcing material and is loaded with polymerization initiator aluminum trifluoromethanesulfonate. Based upon this, a separator‐induced in situ directional polymerization technique is demonstrated, and the extra addition of initiators into liquid precursors is no longer required. The polymerization starts from the surface and interior of the separator and extends outward with the gradually dissolving of initiators into the precursor. Compared with its traditional counterpart, the separator‐induced poly(1,3‐dioxolane) electrolyte shows improved interfacial contact as well as appropriately mitigated polymerization rate, which are conducive to practical applications. Electrochemical measurement results show that the prepared poly(1,3‐dioxolane) solid electrolyte possesses an oxidation potential up to 4.4 V and a high Li+ transference number of 0.72. After 1000 cycles at 2 C rate (340 mA g−1), the assembled Li||LiFePO4 solid battery possesses a 106.8 mAh g−1 discharge capacity retention and 83.5% capacity retention ratio, with high average Coulombic efficiency of 99.5% achieved. Our work may provide new ideas for the design and application of in situ polymerization technique for solid electrolytes and solid batteries. A separator‐induced in situ directional polymerization technique for the preparation of solid polymer electrolyte is demonstrated, and the extra addition of initiators into liquid precursors is no longer needed. The interfacial contact is further improved compared with the traditional method, and meanwhile mitigated polymerization rate is obtained.