The most daunting challenge of solid polymer electrolytes (SPEs) is the development of materials with simultaneously high ionic conductivity and mechanical strength. Herein, SPEs of lithium bis‐(trifluoromethanesulfonyl)imide (LiTFSI)‐doped poly(propylene monothiocarbonate)‐b‐poly(ethylene oxide) (PPMTC‐b‐PEO) block copolymers (BCPs) with both blocks associating with Li+ ions are prepared. It is found that the PPMTC‐b‐PEO/LiTFSI electrolytes with double conductive phases exhibit much higher ionic conductivity (2 × 10−4 S cm−1 at r.t.) than the BCP electrolytes with a single conductive phase. Concurrently, the storage moduli of PPMTCn‐b‐PEO44/LiTFSI electrolytes are ≈1–4 orders of magnitude higher than that of the neat PEO/LiTFSI electrolytes. Therefore, simultaneous improvement of ionic conductivity and mechanical properties is achieved by construction of a microphase‐separated and disordered structure with double conductive phases. Solid polymer electrolytes (SPEs) of lithium bis‐(trifluoromethanesulfonyl)imide‐doped poly(propylene monothiocarbonate)‐b‐poly(ethylene oxide) block copolymers are successfully fabricated. SPEs with simultaneously improved ionic conductivity and storage moduli are achieved by construction of a microphase‐separated and disordered structure with double conductive phases, offering an effective strategy for the design of SPEs with optimized and balanced properties for advanced lithium battery technology.