Short- and medium-span bridges commonly use prestressed concrete (PSC) I-type girders in their construction. Prestressing in traditional PSC girders is usually applied once at the beginning stage of fabrication. The magnitude of the prestressing force is limited within allowable stresses at various construction stages. Consequently, for longer spans, a larger sectional depth is required. To achieve more economical PSC girders in terms of a reduced girder depth for a given span, or a longer span for a given girder depth, the method of multistage prestressing at different loading stages is introduced. Theoretical expressions for stresses at each loading stage are developed and experimental verification is carried out by 2 full-scale flexural tests. From parametric studies and test results, it was observed that the developed girders satisfied the requirements for the strength and ductility, and can be an economical alternative to other types of PSC girders for long-span bridges.