This paper probes the determination of the main gas-generation phase of marine organic mattes using the kinetic method. The main gas-generation phase of marine organic matters was determined by coupling the gas generation yields and rates in geological history computed by the acquired kinetic parameters of typical marine organic matters (reservoir oil, residual bitumen, low-maturity kerogen and residual kerogen) in both China and abroad and maturity by the EasyRo(%) method. Here, the main gas-generation phase was determined as Ro%=1.4%–2.4% for type I kerogen, Ro%=1.5–3.0% for low-maturity type II kerogen, Ro%=1.4–2.8% for residual kerogen, Ro%=1.5–3.2% for residual bitumen and Ro%=1.6–3.2% for reservoir oil cracking. The influences on the main gas-generation phase from the openness of the simulated system and the “dead line” of natural gas generation are also discussed. The results indicate that the openness of simulation system has a definite influence on computing the main gas-generation phase. The main gas-generation phase of type II kerogen is Ro%=1.4–3.1% in an open system, which is earlier than that in a closed system. According to our results, the “dead line” of natural gas generation is determined as Ro=3.5% for type I kerogen, Ro=4.4–4.5% for type II kerogen and Ro=4.6% for marine oil. Preliminary applications are presented taking the southwestern Tarim Basin as an example.