In nuclear power plants, the Boron Recycling System (ZBR) receives drainage generated by changes in temperature and boron concentration of the primary loop, and the degassing tower is used to remove hydrogen and radioactive fission gas from the reactor coolant. In the Hualong nuclear power unit, due to poor steam quality, unstable liquid level in the drain tank, poor efficiency of the drain valve, and delayed response of the drain valve, the pressure difference in the deaerator cannot be established, resulting in the failure of the degassing tower start-up. Improving steam quality is the key factor to determine whether the degassing tower can start successfully and run stably. The direct influence factor of steam quality is steam drain. If the steam quality is improved, and the steam drain can be discharged in time, the pressure of the degassing tower during the start-up process will be more stable, and the success rate of the degassing tower start-up will be improved. In this context, this article optimizes the control of drainage in the degassing tower by changing the type of drain, increasing the volume of the drain tank, and optimizing the PID parameters of the drain valve. The modified water drain control can not only solve the problem of the degassing tower trip caused by water drain, but also effectively improve the steam quality, so as to improve the start-up success rate of the degassing tower, which is of great significance to ensure the safe and stable operation of nuclear power units.