Building sector accounts for nearly 30% of global final energy usage, and its energy demand is expected to keep growing with the increasing population and expanding tertiary industry in the coming decades. To achieve global climate mitigation goal, building sector will need to fully tap its potential on energy conservation and CO2 reduction. This paper simulated the transformation pathways of global energy system under 2-degree and 1.5-degree climate targets with Global TIMES model, classified the 14 model regions into high, middle and low income region, and analyzed the main features and key challenges of each region's building sector transition pathways. Modeling results show that: (1) For global building sector, 32 Gt of CO2 emissions reduction is required between 2010 and 2050 by 2-degree target, additional 28 Gt of reduction would be essential to achieve 1.5-degree target; (2) High income region would take the lead in building sector's CO2 mitigation, while low income region may be increasingly important when the carbon constraint gets more stringent; (3) Electrification is a critical approach in building sector's low carbon transition, by 2050, the share of electricity in final energy consumption is expected to reach 44%, 54% and 59% in reference, 2-degree and 1.5-degree scenarios respectively.