The superior tariff(optimum tariff) is so a kind of tariff, it make one international tradeEasy condition of improvement opposite at its trade quantity of decrease of negative influence of clean income maximize.NamelyWhile taking free trade as point of departure, being a country exaltation its tariff rate, its welfare gradual increment arrive biggestValue(the superior tariff rate), then be tariff rate exceed the superior tariff rate, its welfare again gradualDescend.End this nation again will pass to forbid sex tariff to return to self-sufficient of produce point.