Abstract: Through the determination of flexural strength, compressive strength test and matrix pH value test of glass fiber reinforced cement (GRC) specimens under different ages and ambient humidities, the effects of ambient humidity on the mechanical properties of GRC specimens blended with active mineral admixtures of fly ash and silica fume were researched. The results show that the ambient humidity has a significant effect on the flexural strength of GRC specimens. With the increase of the age, the higher relative humidity is, the more flexural strength of GRC specimens reduces. After 56 d age on the condition of temperature of 60℃ and relative humidity of 95%, the flexural strength and compressive strength of GRC specimens blended with 40% fly ash and 10% silica fume both increase by 48.5% and 23.6% respectively, meanwhile the pH value of GRC matrix decreases by 6% in comparison to those without fly ash and silica fume. On the same condition of ambient humidity, the pH values of specimens blended with fly ash and silica fume are lower than those of ordinary portland cement specimens in every age, which shows that the addition of fly ash and silica fume can reduce the alkalinity of cement hydration liquid phase and slow down the corrosion rate of fibers. So it can dramatically improve the mechanical property and durability of GRC specimens. Based on the analysis of the test results and MATLAB software, the mathematical relationship between the mechanical properties of GRC specimens including the flexural strength and compressive strength and the pH value of cement mortar matrix, the age can be established.