In this paper, UV spectrophotometry was used to test the chromium content of bast fiber of ramie and diphenylcarbazide (DPC) was used as color reagent. The optimal technology was: dropping volume of phosphoric acid 0.4～0.6 mL, DPC content 2.0~4.0 mL and coloration time 5-10 minutes. The results showed that complexes were formed from Cr (VI) ions and DPC under phosphoric acid condition. There was a maximum positive absorption peak at the 540nm. Cr (VI) concentration (in the range of 0.004～1.000 mgL-1) and the absorbance (A) of complex obeyed the Lambert-Beer law.