Flexural equation was established in this paper, which was based on application of deepwater drilling riser and Ritz methods in elastoplastic mechanics. Axial tension, inclination angle, lateral gravity components and axial gravity components were considered. Therefore, simple calculation method for natural frequency of drilling riser was achieved. Free-hinged beam was taken as constraint condition of flexural equation combined with drilling string real conditions, which made research approach practical work more. Purpose of research was to provide reasonable design and selection for deepwater drilling riser, and provide a more efficient and applied method, which would show guiding significance for deepwater drilling operation.