Decarbonization of the power sector is one of the most important efforts to meet the climate mitigation targets under the Paris Agreement. China's power sector is of global importance, accounting for similar to 25% of global electricity production in 2015. The carbon intensity of China's electricity is still much higher than the global average, but the country has made important strides toward a low-carbon transition based on two main pillars: improvement of energy efficiency and decreasing the share of fossil fuels. By applying a decoupling indicator, our study shows that 21 provinces achieved a "relative decoupling" of carbon emissions and electricity production and the remaining nine provinces achieved "absolute decoupling" between 2005 and 2015. We updated China's emission factors based on the most recent data by also considering the quality of imported coal and compared our results with the widely used Intergovernmental Panel on Climate Change coefficients to show the sensitivity of results and the potential error. Our decomposition analysis shows that improvement of energy efficiency was the dominant driver for decarbonization of 16 provincial power sectors, while the access to low-carbon electricity and substitution of natural gas for coal and oil further accelerated their decarbonization.