Terahertz waves, located between the millimeter waves and infrared radiation in the electromagnetic spectrum, have unique penetration and imaging properties for diverse application scenarios. Terahertz computed tomography, derived from its ancestor in the X-ray domain, is known as a major transmissive three-dimensional terahertz imaging method to obtain the inner geometrical structure of the sample as well as its three-dimensional distribution of either absorption coefficient or refractive index. In this study, a three-dimensional printing binary diffraction lens is proposed to generate a convergent terahertz beam with a focal depth of 25 mm and a waist diameter of 1 mm, which enhances the resolution and reconstruction fidelity of a 0.3 terahertz computed tomographic system.For this terahertz binary diffractive lens, its focal depth is modulated by sets of concentric rings with standard step height and different integer multiples of step width. The phase delays caused by steps of binary heights are 0 and π, respectively. A simulated annealing algorithm is applied to find the premium structure of this annular phase modulator, in which the full width at half maximum of the focal spot as well as the maximum available depth-of-focus are selected as the constraints. This diffractive lens is constituted by 20 sets of concentric rings with minimum width of 0.5 mm, with a total effective diameter of 25 mm. It is fabricated by a three-dimensional printer with a precision of 0.1 mm using a photosensitive resin material. Its refractive index and transmissive ratio are 1.66 and 82%, respectively. The step height is therefore set to 0.81 mm, corresponding to an optical path difference of half wavelength. In terms of depth-of-focus, both theoretical value and experimental value are approximately 25 mm, the latter of which is measured by an array microbolometer. The theoretical full width at half maximum of the focus spot is 1 mm while experimental one is approximately 0.8 mm. The disparity comes from incomplete coverage of the incident beam on the effective region of the lens. For comparison, the minimum diameter of the focal spot achieves about 2 mm with a very limited depth-of-focus by using a regular polymethylpentene lens to converge the terahertz beam.A terahertz computed tomographic setup is built based on a continuous-wave avalanche diode source and a golay cell detector. Two different samples are fabricated by three-dimensional printing as well using white resin. Either sample can be mounted on a combination of a manually controlled rotation stage and an electric controlled translation stage, in which the annular and lateral interval are 5° and 0.5 mm, respectively. It is assumed that the terahertz beam propagates in a straight line through the sample, and the sinogram is recorded over a rotation of the sample from 0° to 180°. According to Fourier slice theorem, two-dimensional cross-sectional image can be obtained from the sinogram using the filtered back projection algorithm. Three-dimensional reconstruction can be obtained by stacking the cross-sections at different heights. The experimental results validate that a resin pipe with a wall thickness of 1.2 mm can be reconstructed with an average error of 4% by using the proposed binary diffractive lens, meanwhile solid resin cylinders with diameters of 1 mm, 1.5 mm and 2 mm can be clearly recognized inside a chamber made of the same opaque material. Quantitative comparison is conducted using a more conventional terahertz computed tomographic geometry with the regular lens. The experimental results validate that both resolution and fidelity of the reconstructed images, i.e., two-dimensional sectional images and three-dimensional reconstruction, can be significantly improved by using this convergence method. Considering the flexibility and relatively low cost of three-dimensional printing, the proposed method would promote terahertz computed tomography into real applications of non-destructive testing.