By exposing high silica/phenolic premix at a relative humidly (RH) of 85％ for 24 h,the composite was prepared by hot pressing the premix,and then it was evaluated by non-destructive testing and mechanical property measurement.Experimental results show that there are a large amount of internal defect in composite materials,including 37.4% Class II region and 49.8% Class Ⅲ region.In addition,mechanical properties of the composite decrease dramatically,showing 55% decrease in tensile strength,60% decrease in compressive strength and 15% decrease in shear strength.The formation of the internal defects can be mainly ascribe to two aspects:(1)The volatilization of part of water absorption in the premix forms pores in the composite during the curing process,forming Class II region;(2)The rest water absorption by fiber weakens the interfacial adhesion between fiber and resin matrix and causes the debonding of the interface in serious conditions, resulting in the formation of the Class Ⅲ region.These reasons lead to decreased mechanical properties of composite.