The upper part of 4th Member of Tertiary Shahejie Formation in Dongying Sag is composed of a set of high quality source rocks. The variation of element content and the typical element ratios generally record the characteristics of sedimentary environment. In this paper, the paleosalinity of the upper part of 4th Member was studied on the basis of a geochemical analysis of trace elements, and three assemblies of source rocks with different geochemical characteristics were distinguished by means of cluster analysis. With the ratios of Sr/Ba, Sr/Ca, Fe/Mn and Sr/Cu, the samples of the upper part of4th Member were assigned to three groups with different paleosalinities. It is indicated that the upper part of 4th Member of the Shahejie Formation was deposited under a comparatively dry weather condition. Nevertheless, the lacustrine salinity evidently increased from the shallow water near the lake brim to deep water at the lake center. Interestingly, the salinity of the northern sag is much lower, which may be attributed to the high riverine input from the Chenjiazhuang-Qingtuozi highland. These findings will highlight the studies of high quality lacustrine source rocks in the study area.