The activation of formation water has important domination to the generation, migration, accumulation and loss of oil and gas.And, the evolution of formation water is closely related to the accumulation process of oil and gas. On the basis of analysis of the formation water distribution characteristics of Niuzhuang sag, and under the guidance of fliud dynamics, three types of enclosure correlating with fluid activation are classified through saline enclosure petrography observation. The three main formation fluid active stages which are end of Ed, deposition period of Ng and deposition period of Nm-Quaternary period are classified in respect of homogenization temperature and salinity measurement of salt-water inclusion in sand rock reservoir combined with the study of thermal evolution history of Niuzhuang sag in this paper. The palae-salinity of different formation fluid active stages is recovered by means of fluid inclusions analysis technique, and the palae-pressure of different formation fluid active stages is recovered by fluid inclusions methane content calculation. The palae-salinity of the evening of Ed period increased, and the palae-salinity of Ng period declined. The palae-pressure of the end of Ed period declined, the palae-pressure of Ng period increased. The ancient fluid evolution model and the dynamic model of formation water activation and oil accumulation are accomplished finally.