High-order harmonics generated by the interaction between intense femtosecond laser and matter have the characteristics of high single photon energy， ultrashort pulse duration， and excellent spatiotemporal coherence. It can be used as the tabletop ultrafast vacuum ultraviolet laser source and soft X-ray laser source in laboratory. At the same time， high-order harmonics are used to generate attosecond pulses. The generation of these advanced laser sources has greatly enriched the research tools of material science. In this review article， we introduce the principle， optimization， and application of high-order harmonics of gases and solids.