In recent years, the lateral photovoltaic effect (LPE) with its unique working mechanism has been explored as an indispensable method for position detection applications. A promising platform for developing self-powered position-sensitive detectors (PSDs) is the 3C-SiC/Si heterojunction, taking advantage of its large built-in voltage and capabilities to work in harsh environments. In this work, we further demonstrated the superior performance for position sensing of the 3C-SiC/Si heterojunction by optimizing the diffusion layer thickness. We fabricated 3C-SiC/Si heterojunction devices with different thicknesses and evaluated the position-sensing performance under different lighting conditions. A maximum sensitivity of 603.65 mV/mm was achieved in the device with a 90-nm SiC layer at zero bias. We elaborated the working mechanism by examining the generation and diffusion of charge carriers under nonuniform light illumination. The excellent sensing performance can be attributed to the high resistivity of the diffusion layer and the trapping sites at the heterojunction interface. Our study further establishes the potential of 3C-SiC/Si for optoelectronic sensing and provides useful guidelines for developing ultrasensitive PSD.