Serratia symbiotica is an important symbiotic bacterium associated with aphids. Its interaction with hosts can be categorized into three modes: free-living, facultative symbiosis, and obligate symbiosis, making it a significant model for investigating the evolutionary mechanisms underlying bacterium-insect interactions. This article reviews recent research progress on strain differentiation, lifestyle characteristics, host interaction mechanisms, and functional roles of S. symbiotica. Significant variations exist among different strains in genomic features, host distribution patterns, transmission modes, and impact on hosts, offering novel perspectives for understanding the diversity of insect symbionts and their co-evolution with hosts.