The low Li ⁺ transport and poor interface have consistently been two major impediments to practical applications of Polyacrylonitrile (PAN)‐based composite solid‐state electrolytes (PCPE). In this work, a polymerizable deep eutectic electrolyte is meticulously designed with high fluidity which consists of Poly (Ethylene Glycol) Diacrylate (PEGDA), Fluoroethylene Carbonate (FEC), Succinonitrile (SN) and dual salts (LiTFSI/LiDFOB) to promote Li ⁺ transport and ameliorate the interface of PCPE. Inclusion of PEGDA monomers and FEC alters the crystallinity of SN, enhancing the wettability of thick electrode, and formation of polymeric 3D network from polymerization of PEGDA can anchor SN and suppress the side reactions between SN and lithium metal. Consequently, the modified PCPE exhibit an enhanced conductivity of 4.47 × 10 ⁻⁴ S cm ⁻¹ with Li‐ion transference number of 0.60, and show an excellent lithium stability. LiCoO ₂ (LCO)/SP‐PCPE/Li batteries with higher loading (3–4.4 V, 6 mg cm ⁻² ) can work for over 300 cycles at 0.5 C. Even with an ultra‐high loading of 16 mg cm ⁻² , LCO/SP‐PCPE/Li batteries achieve an excellent cycling performance. This work provides new insights into how to construct a robust interface for solid‐state batteries with high‐loading cathode.