Objective:To analyze the composition, the changes of expense structure and the influencing factors of hospitalization expenses, for reference in optimizing the cost control of day surgery.Methods:Collection of the first page data of patients with the top three diseases(varicose veins of lower limbs, chronic cholecystitis and varicocele)in the day surgery volume ranking in three tertiary general hospitals in a city in 2020. The confounding factors were eliminated through propensity matching. The structural change of hospitalization expenses was analyzed by structural change degree, and the influencing factors of hospitalization expenses were analyzed by grey correlation degree and multiple linear regression.Results:After 1∶1 propensity matching of the first page data of 752 patients with day surgery and non day surgery, 98 patients with lower extremity varicose veins, 356 patients with chronic cholecystitis and 38 patients with varicocele were finally included. Compared with non day hand, the total hospitalization cost of day surgical instruments decreased, and the cost structure changes of chronic cholecystitis, varicocele and varicose veins of lower limbs were 14.59%, 6.20% and 16.20% respectively. Among them, the general medical service fee, nursing fee and examination and laboratory fee showed a downward trend, and the fees of materials and drugs showed an upward trend. General medical service fee, nursing fee, examination and laboratory fee, clinical diagnosis fee, treatment fee, drug fee, material fee and other expenses presented a high correlation with the cost of day surgery(grey correlation>0.90). The payment method, wound healing type and discharge diagnosis can influence the cost of day surgery( P<0.05). Conclusions:Compared with non daytime surgery, the total hospitalization cost of day surgery has a certain cost control effect, but it can not reduce the cost of all projects. The main influencing factors are the internal composition of the cost, payment method and so on. The hospitals should focus on tapping the internal cost control potential of day surgery and further expanding the coverage of day surgery diseases.