Solid polymer electrolytes (SPEs), such as polyethylene oxide (PEO), are characteristic of good flexibility and excellent processability, but they suffer from low ionic conductivity and small Li+ transference number at ambient temperature. Inorganic solid electrolytes (ISEs), garnet-type Li7La3Zr2O12 and its derivatives (LLZO-based) in particular, possess high ionic conductivity at room temperature, wide electrochemical stability window, large Li+ transference number as well as good stability against Li metal anode. Nevertheless, lithium dendrites growth, interfacial contact issue and brittle nature of LLZO-based ceramic electrolytes prevent their practical applications. In response to these shortcomings, LLZO-based/polymer solid composite electrolytes (SCEs), taking complementary advantages of two kinds of electrolytes, and thus simultaneously improving the electrode wettability, ionic conductivity and mechanical strength, have been made to develop high-performance SCEs in recent years. Herein, the intrinsic properties and research progress of LLZO-based/polymer SCEs, including LLZO-based/PEO SCEs (LLZO-based/PEO SCEs with uniform dispersion of LLZO-based fillers and LLZO-based/PEO layered SCEs) and LLZO-based/novel polymers SCEs, are summarized. Besides, comprehensive updates on their applications in solid-state batteries are also presented. Finally, challenges and perspectives of LLZO-based/polymer SCEs for advanced all-solid-state lithium batteries (ASSLBs) are suggested. This review paper aims to provide systematic research progress of LLZO-based/polymer SCEs, to allow for more efficient and target-oriented research on improving LLZO-based/polymer SCEs.