The ejection rate of overflow generated by air-lift pump at reduced pressure has been studied under various operating conditions, namely, top pressure, gas flow rate, nozzle submergence, riser diameter and lift ratio. Slow motion moveis were taken to decide the levels of the experimental parameters. The decrease of top pressure and the incerase of nozzle submergence have the same effect as the increase of gas flow rate on the ejection rate of overflow. The ejection rate of overflow increased as top pressure decreased, nozzle submergence increased, gas flow rate increased, lift ratio decreased and/or riser diameter decreased. The churn-turbulent flow is predominant on flow regime.