Abstract: The public policy evaluation mechanism of trinity combination of "public participation, experts argument and govern-ment guidance"reflects the balance between natures of democracy and science.The balance requires that China's policy evalua-tion theory should achieve objectively breakthrough in the four aspectsc:onstructing the pluralistic and compound subjects of poli-cy evaluation, considering fully the evaluability of public policyr, econstructing the criteria and indicators and the methodology of policy evaluation.