A structure with “changeable boundaries”, changed from construction stage to service stage, is a new design idea used in long span spatial structures. The inner force distribution will be more reasonable and the structure self-weight will be lighter, if the idea is adopted. Adoption of the idea enables the inner force distribution of the structure changing from the construction stage to the service stage. Although the time of the inner force changing is much short, it has a significant influence on the loading behavior and the construction cost of the structure. As an example, a simple beam with different boundaries (unchangeable and changeable), under uniformly distributed lateral loads, is analyzed in this paper. Through static analysis, the loading behavior of the beam under three different boundaries, including simple support, fixed support and changeable boundary, is compared. The linear buckling behavior of the beam with changeable boundaries is also investigated. The critical equilibrium equation is derived subsequently for computing the critical stability load. A new computation method for the beam with changeable boundaries is proposed.