This paper proposes a method to accurately predict thermal errors in spindles by applying experimental modifications to preliminary theoretical models. First, preliminary theoretical models of the temperature field and the thermal deformation are built via mechanism analysis, which is based on the size of the spindle and the parameters of the bearing. Then, thermal basic characteristics tests are conducted at two different initial temperatures. Finally, the results of the thermal basic characteristic tests are evaluated, and the preliminary theoretical model is modified to obtain the final model. A simulation of axial thermal deformation under different speeds is conducted by finite element analysis. It shows that the relationship between the axial thermal deformation and the speed is approximately linear. The model is validated via some experiments on the spindle of a numerical control lathe. The results indicate that the proposed model precisely predicts the spindle’s temperature field and multi-degree of freedom thermal errors.