We report a highly sensitive ion detection method based on an optofluidic catalytic laser, which uses the fluorescent product generated by the enzyme-substrate reaction as the gain medium. Using ions as inhibitors, the catalytic reaction slows down, resulting in a delay in the lasing onset time, which can be used as the sensing signal. The sensing mechanism of the catalytic laser is theoretically analyzed and the performance is experimentally characterized. Sulfide anion is chosen as a model ion because of its broad adverse impacts on both environment and human health. Due to the optical feedback provided by the laser, the small difference in ion concentrations can be amplified. Consequently, a detection limit of 10 nM is achieved with a dynamic range as large as three orders of magnitude, representing significant improvement over the traditional fluorescence and colorimetric methods. This work will open a door to a new catalytic-laser-based chemical sensing platform for detecting a wide range of species that could inhibit the catalytic reaction.