Abstract: A method of acidic dye spectrophotometry coupled with focused microwave assisted extraction was applied to determine the total alkaloids in Crotalaria sessiliflora L. The sample with particle sizes of 0.180 mm was extracted by microwave at middle power level for 50 min with methanol as extract and the material-solvent ratio of 1∶25. The extracted obtained alkaloids were reacted with bromothymol blue to form ion-pair complexes in the buffer solution of KH2PO4-NaOH with pH of 7.2. The complexes were then extracted with chloroform and absorbance was measured at 417 nm. The linearity range of the mass concentration of alkaloids was 0.25-250 mg·L-1 with detection limit of 1.5 mg·L-1. The method was used to measure total alkaloids in Crotalaria sessiliflora L.,giving recovery rates in the range of 106%,with RSD (n=6) less than 3.0%.