American constitutional democracy originates from traditional governance in American colonial period and American Independence Revolution of,develops in political practice of more than 200 years after the founding of the United States,and embodies in the political practice of American federal system,"tripartite-power" system,and two-Party system.Its main characteristic is that a logic approach road of "Three-Step",changing in stability,and attaching importance to universal protection of human rights.It plays a valuable pole in American democracy and development;however,it has lots of defects and deficiencies.