Abstract: On the basis of the research of the petrochemical characteristics of volcanic tuff from Mid-Ordovician series in the southern margin area of the Ordos Basin,the author considers that the volcanic tuff of this area belongs to calc-alkaline rock series and tholeiite series respectively.According to δ-τ index diagrams and other judgement indexes,the author also believes that volcaiiic tuff is an island arc eruption product.This provides evidences forthe further analvsis of tectonic environments in Middle Ordovician epoch.