Based on polynomial s-transfer functions of transient heat conduction through a building construction, a novel and simple model is developed for building thermal load calculation. The polynomial s-transfer functions are estimated from the theoretical frequency responses of the building construction by frequencydomain regression method. The simple polynomial s-transfer functions are completely equivalent to the hyperbolic s-transfer functions in terms of frequency characteristics. First, the frequency responses of the total transmission matrix are calculated within the frequency range concerned. Then, a set of linear equations is solved yielding a simple polynomial s-transfer function for cross and internal heat conduction. Finally, simple recursive formulae are obtained by inverse Laplace transforms and discretization of the convolution integrals. The coecients of the model are independent from time step and the time step can be varied according to the requirement during simulation. Validations and comparisons show that this model has fast computation speed and no numerical unstability, and provides high accuracy and good Β―exibility to the requirement of variable time step.