This paper uses two techniques of isostatic correction and two-dimensional density fitting to analyze Bouguer gravity anomaly in Tibet,and obtains the regional and residual isostatic gravity anomaly in Tibet and two-dimensional density structure along 88.5°E.The results show that the crust and upper mantle beneath northern Tibet are in a deficit state with a lower density than other regions in Tibet.Based on the existing data of regional geology,tectonics,geophysics and geochemical characteristics of magmatic rocks in the northern Tibet,those great eruptions of potassic volcanoes in northern Tibet are the result of break-off of the front of lithosphere mantle of Asia under on-going northward subduction of the Indian lithosphere mantle since ~65Ma.