Abstract: Nowadays, for its outstanding performance on safety features and multiple applications, small modular reactor （SMR） has gained much attention in the field of nuclear energy. Among all of the SMR designs, advanced pressurized SMR has the best maturity, and expected to be the first kind of SMR for commercial deployment. In this paper, the definition and current development situation of SMR were briefly introduced. Five SMR designs with much maturity were also introduced, namely mPower, NuScale, W-SMR, IRIS and SMART. With a method of side-by-side comparison, it is found out that different designs follow same considerations, and there are many similar design improvements that can be categorized as typical SMR design features. Compared with the design of large passive-safety reactor AP1000, the improvements of SMRs on both inherent and engineered safety features were analyzed. By the summary and analysis work done above, a relatively clear understanding of the SMR design is obtained.