The paper has calculated magma density of basaltic rocks from various tectonic settings. It firstly suggested that two types of magma density: I density type of primary basaltic magmas in Arc Regions; Ⅱ density type of primary basaltic magmas in Within Plate and Mid-Ocean-Ridge. The paper points out that Ⅱ magma type concern with the pressure and temperature conditions on which magmas formed, and Ⅰ magma density is controlled by fO2 and fH20 in source of basaltic magmas. So, the density of primary basaltic magmas is a important criteria to distinguish geodynamics settings in which magmas formed. Considing the relationship of density with pressure, the paper suggests that density of primary basaltic magmas controls on the depth of magma chamber and condition of temperature-pressure on which magma crystallized, on the some extent, and make effect on intrusion-eruption of magmas. In the regions of island-arc, magma density and the crust thickness control on time-space evolution of rock series(Ih-CA-Sh) and decide the crystallization condition and evolution path of magmas. Using data about magma density, "two-layer" model of crust structure in Yidun Island-Arc regions，westhern Sichuan Province，has been established.