In order to increase thermodynamic performances of direct heated solar desalination system, thermodynamic analysis is done. The results show that the overall exergy efficiency is very low. The system should be upgraded. Flash temperature and distillation temperature should be justly distributed, and highest seawater temperature can be maximized on request of corrosion control. It is an effective medium to promote exergy efficiency that reasonable setting of temperature difference between shell side and tube side, evaporator can be Improved by transverse tubule and falling film. This means can also promote exergy efficiency. Effective use of the high salinity water and cooling water can raise energy using standards.