In order to solve the fouling problem of pre-security filters of water treatment equipment in power plants,the chemical composition of scale sample were analyzed by energy dispersive X-ray spectroscopy, scanning electron microscopy and transmission electron microscopy. The composition analysis of water source and the compatibility test of chemical agents were carried out. The results showed that the main components of the scale sample were aluminum compounds, which existing in the forms of AlPO4, Al(OH)3 and Al3(CO3)2. The concentration of residual aluminum ion in the influent was 0.34 mg·L-1. Excess aluminum ion reacted with reverse osmosis scale inhibitor and generated white insoluble particulate matters. The turbidity of the solution was positively correlated with the dosage of scale inhibitor. When adding 27 mL scale inhibitor into 200 mL solution, the turbidity of the solution increased up to 4.84 NTU. The residual aluminum flocculants in the influent was incompatible with the scale inhibitor, which would result in fouling.