A new rapid evaluation method of reverse osmosis (RO) scale inhibitors based on pH measurement has been reported. Five scale inhibitors, Amino Trimethylene Phosphonic Acid (ATMP), Hydrolyzed Polymaleic Anhydride (HPMA), GE-Argo, PTP-0100 and TPTO-150, each at the dose of 1 ppm was added for evaluating the performance of CaCO3 scale controlling. Brine pH was the only measured parameter at periodic time intervals. The criterion was that the pH drops, the precipitate occurs. We can compare the efficacy against CaCO3 by calculating the supersaturation value of CaCO3. The test results show that Argo is the best effective inhibitor and HPMA is the worst of all. This method with high sensitivity and accuracy is fit for screening out scale inhibitors before RO dynamic test.