Western Migrants ' Integration Theories mainlyfocus on the categorization and causal explanation on theintegration process of migrants. The Canonical Accountproposed by Robert E. Park and Milton M. Gordon usheresin a new age in this field. Other theories, such as Straight-line Assimilation Theory, Bumpy-Line Theory of Ethnicity,Segmented Assimilation Theory and Multiculturalism, arethe critical developments based on the canonical accounts.These theories undergo the following change: the change ofintegration result from inevitability to relativity, the changeof integration thread from direct one to indirect one, thechange of integration mode from completeness to selection,the change of integration direction from sole one to multipleone. More and more studies tend to emphasize the diversedirections, bumpy-line process and relative outcomes ofmigrants' integration process. Furthermore, the latest trendmainly concentrates on the difference on integration processamong migrants, which is expected to have impact on thetheory reintegration in the future.