This paper analysed the affect of season on mortality in Shanghai County (a rural area) using the Moving Average Method. The advantage of the method is that it can balance the periodical change, long-term change and random change of mortality, so it can analyse the affecting of pure season factor on mortality. The results showed a relationship between mortality and season. The mortality of 0 years old was high in December, possibly due to the cold. The mortality of preschool children was high in summer, farm-busy-time, possible reasons were drowning and parents did not have much time to care for children. The mortality of middle-age, old people, and patients with cardiovascular disease, cerebrovascular disease and disease of respiratory system, was high in winter and summer, possible reason was cold attacks. The mortality of accidental death was high in Summer, possible reason due to drowning.