Abstract: The trace amount SO<SUB>4</SUB><SUP>2-</SUP>&nbsp; in electronic grade phosphoric acid was determinated by spectrophotometry.Experimental result showed that under the wavelength of 460 nm,there was good linear relation between absorbency and content of SO<SUB>4</SUB><SUP>2-</SUP>;and stationary time that absorbency value needed became longer along with the increase of sample quantity of phosphoric acid.When absorbency become stationary,there is good linear relation between the phosphoric acid sample quantity and the content ofSO<SUB>4</SUB><SUP>2-</SUP>.So the content of SO<SUB>4</SUB><SUP>2-</SUP> can be adjusted by turning the sample quantity to make it in scope of standard curve.Mass concentration range of SO<SUB>4</SUB><SUP>2-</SUP> that can be determinated is 2~12 μg/mL,and mass fraction range of SO<SUB>4</SUB><SUP>2-</SUP> is (8~300)×10<SUP>-6</SUP>.Impact of stabilizer on absorbency was also studied.In addition,the recovery rate of SO<SUB>4</SUB><SUP>2-</SUP> was determined and the reliability of this experiment method was checked.