A new multizone model which is an improvement on the state space model is presented, which is potentially more efficient in the simulation of large scale buildings than other methods such as finite difference, transfer functions, or finite volume. The modeling philosophy is firstly discussed. Then the principle and algorithm of the new model are described. Finally, a PC based program BTP developed based on state-of-the-art modeling strategy reveals its applicability with fast calculation speed and satisfactory accuracy in the modeling of building energy performance.