CO2 mitigation in the global energy system is critical in tackling climate change, each region should design its long-term strategies and implement policies accordingly to promote the energy transformation. To evaluate the impacts of existing energy policies and propose possible enhanced NDCs (Nationally Determined Contributions), this paper applied a 14-region global model to explore the transitions of the global and regional energy system. With the modelling of early and late NDCs enhancing plan for 2-degree target, the required energy transition of each plan was analyzed, together with the vital challenges and potential economic impacts. Model results show: 1. Existing energy policies can reduce the annual emission growth rate to 0.6% for 2015 - 2020, while their influence on long-term mitigation is limited; 2. If enhanced NDCs begin from 2030 onwards after realizing current goals, 2-degree target would become quite challenging in later period, some regions will have to cut the energy service drastically; 3. With the assumption of early enhancement and free carbon trading in the cost optimal enhancing plan, the total cost of global energy system could be reduced by 10% in 2050, compared with the former plan; 4. Under this cost-optimal enhancing plan, developing regions may face great challenges in short to medium term, 122% of additional investment should be put In power sector in 2030, international support on both technology and finance would be essential.