In this paper, a whispering-gallery-mode (WGM) oval-shaped microcavity based on polymers was proposed, breaking the circular symmetry of traditional microcavities, and its wavelength-level notch on the boundary achieved unidirectional emission at an extremely low far-field divergence angle. In addition, the three-dimensional finite-difference time-domain method was used to investigate the WGM and unidirectional emission performances of the proposed microcavity, especially the influence of different deformation parameters on the far-field properties. With the optimal deformation parameters, the WGM near- and far-field performances of light field distribution near the wavelength of 536 nm were obtained. Besides, the effect of notch size on the output spectrum of the proposed microdisk was also analyzed. Then, extending the oval microdisk to an ovoid microsphere further reduced the beam divergence (~2.5°), with the highest unidirectional emission efficiency being 93%. Therefore, the proposed structure can function well as an excellent platform to explore the light-matter interaction, expand the research range of notched microcavities in the field of unidirectional emission, and promote the development of photonic microdevices in the fields of biomedicine and environmental detection.