A comparison of the existing sound source model for calculating transfer matrix of two-dimensional rectangular acoustic cavity is performed in this paper. Based on the comparison, a new square linar sound source model is developed to calculate the pressure response function. Results show that this new source model can not only overcome the singularity problem of the point source model but also get more uniform pressure distribution than that of the surface source model provided that the size of the new model’s geometry is reasonably controlled. On the other hand, the mathematical model of the new square line-source is simpler than that of the circular line-source model. Therefore, it can improve the computational efficiency significantly. Finally, the feasibility of the new source model is substantiated by performing some numerical experiments.