Hydrocarbn explusion threshold (HET) is the critical condition for hydrocarbon expulsion in separate phase from a source rock when the generated hydrocarbon amount has satisfied all needs for ahsorpBon by minerals, solution in water, and blocking of capillary pressure. Research results show that the HET varies mainly with three geological parameters: total organic carbon content (C%), kerogen type index (KTI) and thermal maturation degree (R0). Source rocks with low C% and KTI cross the HET at a high level of maturation degree (larger R0); source rocks with lower R0 and C% can also cross the HET if the kerogen has a larger KTI. Under general geological conditions, a source rock first crosses the methane expulsion threshold (HETgl), then the heavw hydrocarbon gas threshold (HETgn), and finally the liquid hydrocarbon expulsion threshold (HET0). In this paper the concept of HET, and its critical conditions, are applied to establish the scientific validity of the concept and grade the source rocks, to study the phases of hydrocarbons in migration and the mechanisms of hydrocarbon acvumulation, and to divide the hydrocarbon expulsion into stages. Applications to different basins in China show that HET provides an accurate and efficient method to guide oil and gas prospecting.