We proposed for the first time that the angle between the projection of the magnetic field line and the sunrise line (AMFS) is a crucial factor controlling predawn heating. Then, we quantitatively investigated the relationship between the predawn heating effect and the AMFS depending on the model results and examined the influence of the length of the magnetic field line (LMF). The results indicate that the predawn heating is influenced by the combined effect of the AMFS and the LMF. Our study suggests that the increase of AMFS promotes predawn heating, while the increase of LMF blocks predawn heating. Finally, we found that when the LMF is about 4000 km and the AMFS is around 30 degrees, the combined effect of the AMFS and the LMF on the predawn heating effect reaches its maximum, exceeding 400K, while the influence of both AMFS and LMF exhibit saturation effects.