A terahertz beam-splitter structure with polyester films as the support based on ultrathin metallic films is proposed, and the splitting model based on the transmission matrix theory is built. The influences of the metallic Cr films and the polyester films with different thicknesses on the reflectivity, transmissivity and splitting ratio of this ultrawide-band terahertz beam-splitter are also analyzed. The theoretical results show that the splitting performance of the proposed terahertz beam-splitter is relatively sensitive to the thickness of metallic Cr films but insensitive to the thickness of polymer films. On this basis, the terahertz beam-splitters with different thicknesses of metallic Cr films are fabricated and tested experimentally. The experimental results are consistent with those from theory analysis. The structural parameters of this device with a splitting ratio of 1∶1 are determined.