Thin-walled structures are commonly utilized in aerospace and aircraft structures, which are prone to buckling under axial compression and extremely sensitive to geometric imperfections. After decades of efforts, it still remains a challenging issue to accurately predict the lower-bound buckling load due to the impact of geometric imperfections. Up to now, the lower-bound curve in NASA SP-8007 is still widely used as the design criterion of aerospace thin-walled structures, and this series of knockdown factors (KDF) has been proven to be overly conservative with the significant promotion of the manufacturing process. In recent years, several new numerical and experimental methods for determining KDF have been established, which are systematically reviewed in this paper. The Worst Multiple Perturbation Load Approach (WMPLA) is one of the most representative methods to reduce the conservatism of traditional methods in a rational manner. Based on an extensive collection of test data from 1990 to 2020, a new lower-bound curve is approximated to produce a series of improved KDFs. It is evident that these new KDFs have an overall improvement of 0.1-0.3 compared with NASA SP-8007, and the KDF predicted by the WMPLA is very close to the front of the new curve. This may provide some insight into future design guidelines of axially compressed cylindrical shells, which is promising for the lightweight design of large-diameter aerospace structures.