Influenced by traditional idea of unification, Emperor Xiaowen expressed his aspiration on unifying China once and again. Including the implement of "using southern expedition to promote southern migration" strategy, Emperor Xiaowen had launched four large-scale southern expeditions in succession. It broke the pacific relations between Northern Wei and Xiao Qi. Although Emperor Xiaowen's four southern expeditions did not unify China, the North exceeded the South in politics, economy, military and culture through four cumulative, progressive and interlocking military operations. It means the balance mechanism inclined at the side of the North.In which the Northern finally exceeded the South in the comprehensive national power. It was the tipping point of changing North-South confrontation into the Northern unipolar status, or “Beiqiang Nanruo”北強南弱 in Chinese term.This paper tries to talk about several issues like the evolvement of Emperor Xiaowen's ideas about the South, Northern Wei's temporal political situation and the four-times warfare conditions,as a way to reveal a historical experience: although it seems that Southern and Northern Dynasties appeals to be superficially complicated, there is still an unification trend brewing in the process of history. Key Words: Southern and Northern Dynasties, Northern Wei and Xiao Qi, Emperor Xiaowen's four expeditions, north-south relation, unification