Optical isolators are essential devices that enable light propagation in a single direction, finding widespread applications in fields such as optical communication and optical computing. A prerequisite for these devices to achieve optical nonreciprocity (ONR) is the violation of Lorentz reciprocity. Traditional approaches to ONR rely on external magnetic fields, spatiotemporal modulation, or nonlinear optical effects. In this work, we proposed a novel, to the best of our knowledge, simple all-optical isolator base on spatial self-phase modulation (SSPM). Through both theoretical analysis and experimental validation, we demonstrate that SSPM induces a non-reciprocal spatial distribution of light beams, providing a foundation for a highly implementable and efficient optical isolator design. The proposed isolator consists of only four basic optical elements: an aperture, a nonlinear medium, and two lenses with different focal lengths. Using zinc selenide (ZnSe), a widely available nonlinear optical medium, the device exhibits remarkable ONR performance at 800 nm such as a high isolation of 25.1 dB along with a minimal insertion loss of 2.7 dB. To further showcase the versatility of this approach, we designed an on-chip optical isolator utilizing an epsilon-near-zero (ENZ) thin film with a large third-order nonlinear refractive index, highlighting the potential of our strategy for integration into photonic circuits. Our approach to optical isolation is exceptionally simple yet highly versatile, making it a promising candidate for a wide range of commercial and industrial applications.