Errors in strain measurements in stereo-digital image correlation (stereo-DIC) caused by camera self-heating have been experimentally observed in previous research, and have been shown to widely vary from one system configuration to another. Such "thermal errors" are sometimes so large that they strongly compromise the accuracy of the measurements. Despite correcting such errors is crucial when aiming at high-accuracy measurements, the mechanism of the thermal error generation and how it relates to the camera parameters in stereo-DIC are still not clear. In this paper, we first explain in detail how self-heating can introduce large artifacts in the strains measured by stereo-DIC. Using a simplified stereovision model, we provide the main equations that describe the theoretical errors in 3D coordinate reconstruction and 3D deformation measurement. Through several sets of simulations, the effect of camera self-heating on the 3D coordinate, displacement and strain measurements, and the effect of camera parameters on the thermal errors in stereo-DIC were explicitly presented based on the derived theoretical formulas. Finally, several real self-heating tests using a smartphone-based single-camera stereo-DIC system confirm the correctness of theoretical analyses and simulation results.