Objective: To explore the clinical expanded application value and compatibility rationale of the modified Chaihu Guizhi Decoction (Bupleurum and Cinnamon Twig Decoction) in the treatment of coronary heart disease, chronic urinary tract infection, and posterior circulation ischemia. Methods: 3 patients diagnosed respectively with coronary heart disease, chronic urinary tract infection, and posterior circulation ischemia were selected. Based on the core effects of Chaihu Guizhi Decoction—harmonizing Shaoyang, regulating the triple energizers, relieving muscle surfaces, harmonizing nutrient qi and defensive qi, supporting yang to transform qi, and coordinating yin and yang—modifications were made according to the pathophysiological characteristics of each patient. Clinical efficacy was observed, and the compatibility logic between the pathogenesis and formula was analyzed using traditional Chinese medicine theory. Results: After treatment with the modified Chaihu Guizhi Decoction, clinical symptoms significantly improved in all three patients, relevant auxiliary examination indicators recovered or improved, and no significant recurrence was observed during follow-up. Conclusion: Chaihu Guizhi Decoction is not only suitable for treating the combined pattern of Taiyang and Shaoyang disorders but also effective for cardiovascular, urinary, and neurological diseases characterized by liver qi stagnation and qi stagnation, disharmony between nutrient and defensive qi, complicated by phlegm-stasis obstruction and deficiency of healthy qi. Modified application based on pattern differentiation yields good results, reflecting the essence of TCM treatment based on pattern differentiation known as treating different diseases with the same method.