The control of water content in the rectification system of butadiene unit is a difficulty in the startup operation of the unit, especially during the start-up of unit after passivation when the time required for water content to reach the standard is long and the startup cost is high. The schemes for water content control, startup drying and startup water content adjustment of the rectification system are optimized. Through the long-term operation practice of the unit, the stable and controllable water content of butadiene product is ensured, and the operational efficiency of the unit is improved.