Power sector faces serious challenges, e.g. greenhouse effect, acid rain, supply security, etc, along with the durative price rising of fossil and more worsening of environmental problems. An assessment model of power mix is put forward in this paper, basing on multi-criteria Diversity Analysis (MCDA). The efficient frontier can be gained at the basis of analyzing the power mix’s risk factors, such as environment pollution, supply security, system stability and land use. By the assessment model, the author analyzes the efficiency of China’s power mix in 2008 and 2020. We can find that actual China’s power mix is non-efficient, because of excessively depending on coal-fired plants. In order to decrease the risk of power mix, China should reduce the proportion of coal-fired plants, accordingly increase the proportion of clear energy, such as hydro, nuclear energy, wind energy, etc, in power mix. Hydro is the first choice for China to improve the efficiency of power mix. Nuclear is second at present. However, wind will become the second choice in future, if its cost can decrease because of scale effect.