Abstract: Galerkin method is a numerical method widely used in mathematics, physics, and engineering problems. There are some controversies among textbooks and some literature about the selection of trial functions. To make an example, this paper uses Galerkin method to solve the static and dynamic problems of a cantilever beam under the axial load. It is proved that correct results cannot be obtained when choosing trial functions that satisfy only displacement but not force boundary conditions, even if increasing the number of trial functions.