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(R2 = 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.