The thermal insulation of a bedding system is one of the most critical factors affecting sleeping thermal comfort. This study reported a mathematical model to evaluate both local and total thermal insulations of a bedding system. To determine the geometric parameters in the model, the geometric model of the bedding system was developed using a 3D virtual simulation program. Its reliability was validated by comparing it with the 3D scanning model. The predicted local and total thermal insulations of bedding systems were compared with those measured by the thermal manikin obtained in a previous study. The bedding systems included six down quilts with different filling weights and involved three body postures. The results showed that the predicted thermal insulation values agreed well with the experimental values. The predicted local and total thermal insulations were with acceptable accuracy, whose errors were within 20% and 10%, respectively. Finally, the research discussed the effects of two main parameters (i.e., the proportions and partial thermal resistances of heat transfer parts) on bedding thermal insulations and provided practical suggestions for regulating bedding thermal insulation. This study has important implications for evaluating the thermal comfort of the bedding system and contributes to improving the sleeping environment.