The invention discloses a method capable of simultaneous determination of various trace anions in water vapor of a thermal power generating unit; the various trace anions in the water vapor of the thermal power generating unit are determined by a capillary ion chromatography, wherein the trace anions comprise F<->, Cl<->, SO4<2->, Br<->, NO3<->, NO2<->, PO4<3->, CH3COO<->, HCOO<-> and C2O4<2->, and the method comprises the following specific steps that a to-be-detected water sample is filtered with a 0.45 [mu]m membrane filter and then is injected into a sample introduction quantitative ring of an ion chromatographic instrument; ion chromatography detection conditions comprise an anion analytical column IonPac AS18 Capillary and a guard column IonPac AG18 Capillary are used, the column temperatures of the anion analytical column and the guard column are 30 DEG C, an eluent solution is an KOH solution, gradient elution is carried out, the suppressing current of a suppressor is 11 mA, the flow rate is 0.010 mL/min, and the injection volume is 10 [mu]L. The method is fast in analysis speed, accurate and reliable in detection results, and suitable for determination of the contents of trace inorganic anions and small-molecular organic acids in the water vapor of the thermal power generating unit.