BEIJING—China’s leaders moved to dispel perceptions of disarray in the management of the world’s second-largest economy, firing a top regulator and setting out specifics on how they intend to restructure the economy without having growth fall off a cliff.

The departure of Xiao Gang as head of the China Securities Regulatory Commission, first reported by The Wall Street Journal on Friday, was announced by the official Xinhua News Agency on Saturday morning. In a brief, Xinhua said the decision was made by the State Council,...