Based on the new trend of world energy development in 2050 and the reality of China's development, how to realize energy transition and ensure oil and gas demand is a major forward-looking issue concerning China's national security. This study simulates the changes in the quantity and structure of primary energy of China based on the policy scenarios of key industries. And it also analyzes the demand of oil and gas in China and its oversea dependence in 2050. The results show that: (1) By implementing the active energy transition policies, the value of primary energy consumption is expected to peak in 2040, with a maximum value of 5755 to 7000 mtce. For different types of energy, coal consumption can reach its peak by 2030, oil consumption will peak in 2040 under both transition scenarios, and natural gas consumption will peak in 2035 under accelerated transition scenario. (2) From the perspective of advancing the transition of the energy structure, China's coal consumption will account for 21% of the total energy amount in 2050 under the transition scenario, and coal will account for less than 10% of total energy consumption by 2050 under the accelerated transition scenario. In both of the transition and the accelerated transition scenarios, oil and gas consumption will account for 30% of China's total energy amount in 2050. If a more active transition policy is implemented, no-fossil energy will be the most important energy sources for China in 2050 in the accelerated transition scenario. (3) High demand and relatively low domestic production of oil/gas will lead to a high level of China's oversea dependence in a mid- and long-term trend. Therefore, the supply of oil and gas to the national energy security in 2050 cannot be ignored based on the policy scenarios of key industries. This research can provide a scientific basis for the policy making of China's energy security.