Downstream of Ma river is the region of developed agriculture, industry, tourism and cultural centre of Thanh Hoa province in particular and of Northern Central region in general. During the recent years, the region has been facing the deterioration of the low-flow and deep saline intrusion in the dry season, meanwhile, hydro-power reservoir system has been constructed and completed, and it is expected that there will be significant impacts. This paper is to briefly introduce some results of researching the effects of reservoir - operation on the saline intrusion, based on that, mitigation measures will be given suitable solution in the future.