The steam ejector is a key component of the low-temperature multi-effect distilled seawater desalination system (MED-TVC). To improve the efficiency of the system, the steam ejector has been optimized and analyzed for its structure. The influence of mixing chamber diameter on ejector ratio is studied using the computational fluid dynamics model (CFD) technology during the process of the ejector structure optimization, and the quadratic function relation between the mixing chamber diameter, the entrainment ratio, and the critical back pressure are obtained. The results show that the increase of mixing chamber diameter will lead to the increase of entrainment ratio and the decrease of critical back pressure. When the primary flow pressure and the secondary flow pressure are fixed at 600 and 15 kPa, respectively, by increasing the diameter of the mixing chamber from 15. 8 to 20. 35 mm, the ejection ratio can be increased from 0. 485 to about 0. 746, and the performance can be increased by 53. 8%. The water production ratio can be increased from 7. 425 to 8. 73, and the percentage increase is about 17. 58%. The optimized steam ejector structure can effectively increase the daily water output of the seawater desalination system, improve system operation efficiency, and reduce energy consumption.