Abstract: Abstract: Fiber-reinforced composites made up of unsaturated polyester resin (UP) matrices reinforced with glass, carbon and aramid fibers were commonly standard structural materials in engineering fields. In recent years, basalt fibers, have recently gained an increasing attention as possible replacement of the conventional glass or carbon fibers due to their advantages in terms of environmental-friendly, cost-effective, high chemical and physical properties. In order to verify the mechanical properties of basalt fiber fabrics/UP composites, the author used the artificial simulating climate box (UV and Condensation) to accelerate aging process of composites. The methods tested accurately the working life of composites and observed color change of fiber surface in different aging time (0, 3, 10, 13, 23 and 30 d). Variation of mechanical properties, microstructure, functional group were analyzed in detail by mechanic testing machine, scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR) respectively. The results showed that with the increase of aging time, the color of fiber surface became light yellow and the mechanical properties decreased gradually. Tensile strength and elongation at break of composites were slowly falling in 3 aging days, followed by significant decline after 10 days. Eventually, there were almost little change in the rest of aging time. Tensile strength and elongation at break of composites had decreased 35% and 20% compared with that of composites before aging time. A similar trend could be seen in flexural strength and modulus of elasticity of composites. However, they were more seriously than tensile strength and elongation at break and had decreased 60% and 52% compared with that of composites before aging time. The results of SEM indicated that resin wear around fiber, crack and fragment under the external force, due to degradation of matrix leading to interface failure between basalt fiber and UP. The author analyzed the mechanism of degradation and proposed the hypothesis that matrix had degraded and produced small molecule compound and radical after aging time. The FTIR results showed that Carbonyl and double bond or carbonyl conjugate system of UP were changed, and ester carbonyl was decomposed to release carbon dioxide. The peak at 1 725 cm-1 was the characteristic peak of C=O and absorption peak weakened. In addition, both C-O-C appearing at 1 280 and 1 130 cm-1 of ortho-phthalic-1, 2-disubstituted at 747 and 702 cm-1 disappeared. Meanwhile, cross-linking reaction such as molecule chains breakage and free-radical termination occurred in composites. This research method rapidly and accurately determined the working life and improved the aging resistance of basalt/flax fiber reinforced UP composites.