Coupling a smartphone-based stereo-DIC system using a single smartphone and a 3D-printed four-mirror adapter has been recently proposed by the authors as a costeffective and field-portable 3D displacements measurement tools. However, this system is found to be very sensitive to temperature variation inside the smartphone and the ambient temperature variation around the system, which further can introduce displacement and strain errors into the measured results. Interestingly, the image coordinate drifts of image points that are directly associated with these thermal errors cannot be well explained by the existing temperature-induced camera deformation models. To eliminate the large thermal errors and realize high-accuracy 3D deformation measurement, a comprehensive thermal error analysis is performed for the smartphone-based single-camera stereo-DIC system. The theoretical estimations based on the existing temperature-inducedcamera deformationmodel are compared with the experimental observations.Then, all the potential error sources that can introducethermal errors in the smartphone-based single-camera stereo-DIC system are analyzed and discussed. Several comparison tests confirm that the thermal errors are caused by both the inner structural changes and the deformation from the adapter, and that the thermal errors from the adapter aremuch larger than those caused by the inner deformation inside the smartphone camera. Finally, amodified mirror adapter was fabricated and proven less sensitive to the temperature variation. This research is expected to explain all the potential thermal error sources in the smartphone-based single-camera stereo-DIC system and further contribute to the correction of the thermal errors and the optimal design of the adapter.