While high density turbidity current and sandy debris flow are two significant geological agents in deep water environment, a dispute always exists in how to distinguish between the two kinds of fluids and their deposits. By discussing the relationship between the fluids, it is of help to clarifying deep water sedimentary process, to properly distinguishing between their deposits and to predicting reservoir. On the basis of reviewing concept, characteristics and concrete examples, fluid properties and sedimentary characteristics are compared. Combined with triggering mechanism, forming process and influencing factors, the relationship between high density turbidity and sandy debris flow is discussed from the two aspects of basic theory and practical application. High density turbidity current deposits can be divided into bottom part, middle part and upper part vertically, in coarsing upwards then fining. Abundant in sand, sandy debris flow deposits are characterized by massive bedding and can be formed by vertically superimposed massive sandstones. Triggering mechanisms of debris flow are various. Besides, in comparison with moderate to weak viscous ones, strong viscous debris flow is not likely to convert in turbidity. In the use of terms, high density turbity is more suitable. If fluid conversion occurs in debries flow, so does high density turbidity current deposits. If it does not happen and source is rich in sand, sandy debris flow deposits can be formed. Sandy debris flow can be used to explain abrupt contact between massive sandstone and overlying muddy sediments, while high density turbidity current can explain gradual contact.