Milankovitch Theory shows that glacial-interglacial cycles in the Quaternary are related to the variation of solar insolation forcing linked to the earth’s astronomical parameters.However,the summer insolation at northern high latitudes,usually considered as the main external forcing for the ice age as Milankovitch pointed out,is marked by the 19- and 23-ka precession periodicities,which is not consistent with the glacial-interglacial cycles.On the other hand,recent studies indicate that the annual mean insolation dominated by the obliquity is also an important external forcing in glacial cycles.In this paper,it is assumed that the glacial oscillation is regulated simultaneously by the annual mean insolation at northern high latitudes related to obliquity and the noon insolation at summer solstice at the ice-line latitude related to precession. Only when both of them exceed some thresholds,does the ice sheet start to melt continuously to induce the global warming and deglaciation.Furthermore,the periodicity transition of glacial cycles at mid-Pleistocene was a natural change when the ice sheet thickness altered.A conceptual model based on this idea gives encouraging results compared with the isotopic data. Milankovitch Theory shows that glacial-interglacial cycles in the Quaternary are related to the variation of solar insolation forcing linked to the earth’s astronomical parameters. However, the summer insolation at northern high latitudes, usually considered as the main external forcing for the ice age as Milankovitch pointed out, is marked by the 19- and 23-ka precession periodicities, which is not consistent with the glacial-interglacial cycles. On the other hand, recent studies indicate that the annual mean insolation dominated by the obliquity is also an important external forcing in glacial cycles.In this paper, it is assumption that the glacial oscillation is regulated simultaneously by the annual mean insolation at northern high latitudes related to obliquity and the noon insolation at summer solstice at the ice-line latitude related to precession. Only when both of them exceed some thresholds, does the ice sheet start to melt continuously to induce the global warming and deglaciation. Furth ermore, the periodicity transition of glacial cycles at mid-Pleistocene was a natural change when the ice sheet thickness altered. A conceptual model based on this idea gives encouraging results compared with the isotopic data.