Hairspring is a fine spiral spring, which is the key component in mechanical watch movement for timekeeping. According to literatures, there are only few studies on hairsprings or spiral springs. Using the Castiliago’s method, the mechanics of hairspring is studied in details. And Computer simulation with Matlab® and experimental validation are also conducted to show the spring frequency. Experiment shows that the theoretical analysis and computer simulation can be used to guide and facilitate the design of hairspring or spiral spring.