Terahertz beams are a kind of Gaussian beam with a big divergence angle and its wave front cannot be simplified to a plane or a spherical wave. All the theoretical derivations from the classical electromagnetic theory and ABCD laws support the following conclusions: firstly, a positive lens converges the terahertz beam in front of the focal plane instead of on the focal plane; secondly, a negative lens which matches the radius of the Gaussian beam’s wave front is more appropriate for terahertz beam collimation. Experiments show that a negative lens with f′=-188 mm at the matching position, z=100 mm, can improve the terahertz beam collimation from 6° to 0.1°. And 20-m terahertz space transmission was realized with a very simple optical scheme.