Abstract: The reason of cracking and the durability problem in 30-year-old Pier of a sea crossing bridge in coastal areas of the Pearl River Delta was studied by means of petrographic analysis, X-ray diffraction analysis, chemical analysis, scanning electron microscopyPenergy dispersive analysis and other techniques. The results showed that there were active fine aggregates in the concrete, and alkal-i silica reaction had occurred. the concrete had been eroded by magnesium salt in sea water and the sediments on the surface of the crude cracks contained a lot of magnesium hydroxide. The cracking in the concrete was caused by the compound damage of alkal-i aggregate reaction and magnesium salt erosion. This is the first report in our country that the compound damage to the concrete in marine environment was caused by the alkal-i aggregate reaction of fine aggregate and erosion of the magnesium salt. It was demanded strongly that the alkaline activity of aggregates be detected and the compound damage of the marine environment be generally considered in the ocean concrete projects. The influence of fine aggregates having alkaline activity on the durability of concrete should be taken seriously.