The phenomenon of photorefractive asymmetrical self-defocusing induced by thermal self-focusing is observed when a Gaussian beam at 532nm with high enough intensity is focused onto a KNSBN crystal. The spatial distribution and physical properties of the asymmetrical self-defocusing are different from those of stimulated noise scattering. It is found that asymmetrical self-defocusing has no contribution to self-pumped phase conjugation. The diameter of the beam when asymmetrical self-defocusing first appears in the crystal is estimated.