The effect of temperature on the transfer function and space intersection angles of a three-line mapping camera was analyzed and the thermal control index of the three-line mapping camera was calculated by a thermal-optical method. First, the temperature field and thermal elastic deformation were analyzed with Finite Element Method(FEM) on the basis of hypothetic thermal loads and the wave surface fitted with a Zernike polynomial was put into an optical software to explore the effect of temperature on the optical transfer function. Then, the temperature regressive data of transfer function for the mapping camera were obtained in an assumed temperature field. Furthermore, the thermal size stability of a mapping base was analyzed and the effect of temperature on the intersection angles of the three-line mapping camera was gotten. Experiments show that the over or short thermal control designs can be avoided with the proposed method and it can provide reasonable data for thermal control indexes.