This paper discusses the cause of large-scale smog and puts forward somerecommendations in the view of energy transition and capability recovery of ecological self-purification. The paper believes that large-scale smog can be attributed to two factors：the first is that total amount of polluting emission has surpassed the threshold that can be absolved by environment，and resulting from the poor environmental standards and unclean energy mix；the second is that the eco-environmental system has lost its function of self-purification because the serious damage since a long time ago. The first recommendation is to transform the energy mix and to develop more clean energy such as natural gas；the second is to improve emission standards and enforce them strictly；the third is to protect eco-environmental system and improve its function of self-purification；the forth is to improve the standards of urban design and optimize the layout.