A novel potassium ions sensor based on mode-filtered light detection was reported.The analyzer was consisting of an optical fiber immobilized with a dye of bromocresol green and a fused-silica capillary.It was found that mode-filtered light intensity decreased with the concentration of potassium ions and a linear detection range of 0.25-20 mmol/L(R~2 = 0.9977) was obtained with a detection limit of 9×10~(-5) mol/L as well as fast response,good reproducibility and reversibility in the working concentration range. A novel potassium ions sensor based on mode-filtered light detection was reported. The analyzer was consisting of an optical fiber immobilized with a dye of bromocresol green and a fused-silica capillary. It was found that mode-filtered light intensity decreased with the concentration of potassium ions and a linear detection range of 0.25-20 mmol / L (R ~ 2 = 0.9977) was obtained with a detection limit of 9 × 10 ~ (-5) mol / L as well as fast response, good reproducibility and reversibility in the working concentration range.