A novel model is developed for calculating transient thermal load through a building multilayer construction. The theoretical frequency responses of cross and internal heat conduction are calculated within the frequency range concerned. Two polynomial s-transfer functions are then estimated from the frequency responses. Finally, some simple recursive formulae are obtained simply by inverse Laplace transform and discretizing. The validations and comparisons show that this model provides fast computation speed, high accuracy, good flexibility to variable time steps and has no numerical unstability.