Abstract: Solid-holdup, which is significant to the design and optimization of reactor, is an important parameter to assess solid suspension in stirred tank. In this work, solid-holdup in gas-liquid-solid system was measured by detected hydrophone signals combined with the partial least squares (PLS) analysis. It was revealed that a characteristic frequency band of hydrophone signals representing the particle motion was in accordance with the results of FFT analysis. A regression model between solid-holdup and hydrophone signal was built up. Comparing the real values with predicted results, it shows that the correlation coefficient (R) is larger than 0.89, the average relative deviation (ARD) is less than 1.02% and the root mean squared error of prediction (RMSEP) is less than 1.73%. Good precision of the prediction model has been verified. It is concluded that hydrophone is feasible for determining solid-holdup in stirred tank and it have the merit of being sensitive and accurate.