An ocular optical system configured to allow imaging rays from a display frame to enter an observer's eye through the ocular optical system to form an image is provided. The ocular optical system includes a lens element having an eye-side surface and a display-side surface. The lens element has an optical axis extending from a display side toward an eye side. The display-side surface of the lens element adopts a Fresnel lens design. The display-side has a plurality of effective sub-surfaces and a plurality of ineffective sub-surfaces. The effective sub-surfaces are configured to allow the image rays to form an image. Each ineffective sub-surface connects two adjacent effective sub-surfaces. The ocular optical system satisfies: 1.500≤R1/SagI≤4.000, where SagI is a total of lengths of orthogonal projections of the effective sub-surfaces respectively projected onto the optical axis, and R1 is a half of a clear aperture of the display-side surface.