Primary sarcopenia (PS) is an age-related degenerative disorder characterized by progressive loss of skeletal muscle mass and function. This review delineates three mechanisms whereby gut dysbiosis drives PS pathogenesis: decreased secondary bile acids inhibit farnesoid X receptor signaling, thereby attenuating muscle protein synthesis; disrupted short-chain fatty acid metabolism weakens free fatty acid receptor 2/adenosine monophosphate-activated protein kinase signaling, aggravating proteolysis and mitochondrial dysfunction; gut barrier impairment activates the endotoxin–Toll-like receptor 4-mediated inflammatory cascade, accelerating ubiquitin-proteasome system activation. Interventional evidence confirms that microbiota-targeted therapies (probiotics regulating bile acid metabolism and prebiotics enhancing short-chain fatty acid production) effectively improve muscle function. By synthesizing molecular evidence of the “gut-muscle axis”, this review offers theoretical references for developing PS prevention and treatment strategies.