Overview: Terahertz detector is an important device in the field of terahertz technology, and it is important to improve its sensitivity. The sensitivity of the detector can be improved in two aspects: one is to further optimize the antenna of the detector, and the other is to optimize the size of the detector and the spot size of incident terahertz wave. Due to the long wavelength of the electromagnetic wave in the terahertz band, the spot size is much larger than the effective acceptance area of the detector, which limits the effective absorption rate of the detector to the incident terahertz wave. In order to make the focus spot to be small, the lens aperture needs to be increased. At present, the commonly used method is to integrate the hyper-hemispheric silicon lens with the terahertz detector to reduce the spot size by one order of magnitude and increase the electric field energy density. However, hyper-hemispheric silicon lens is difficult to be ultra-thin and ultra-light, and is not planar, which is not conducive to the device integration, especially for array detectors. In this paper, a series of metasurface lenses for terahertz detectors are designed using sub-wavelength silicon cylinders. By tuning the diameter of the silicon cylinders, the transmission phase of the terahertz wave can be controlled from 0 to 2π with high transmission amplitude. At 1 THz, the backside integration of the designed single-surface lens with the terahertz detector can increase the electric field energy density in the core region of the THz detector to 32 times that of the incident plane wave, and reduce the focal spot to the same order of magnitude as the wavelength. Based on the feasibility of fabrication and anti-reflection considerations, we propose a two-sided metasurface lens, which further increases the energy density of the electric field to 44 times that of the incident plane wave. Compared with the traditional hyper-hemispheric silicon lenses, the size and thickness of the metasurface lens are smaller and more convenient for integration. Metasurface lenses have a great prospect for reducing the complexity of the terahertz system and improving the responsiveness of the detector, and provide a new idea for the integration and miniaturization of the terahertz device. However, the current metasurface lenses produce many side lobes after focusing, resulting in low focusing efficiency. Further research needs to further optimize the materials and unit structures of the metasurface lenses, to improve the focusing efficiency and electric field energy density.In this paper, a focusing lens for terahertz detection is designed using a metasurface composed of sub-wavelength silicon cylinders. By tuning the diameter of the silicon cylinder, the transmission phase of the THz wave is controlled from 0 to 2π. At 1 THz, the terahertz electric field energy density focused by the single-sided metasurface lens designed can be increased to 32 times that of the incident wave. After adding the anti-reflection, a double-sided metasurface lens is proposed, which is feasible in processing, increasing the electric field energy density to 44 times that of the original. Compared with the traditional hyper-hemispheric terahertz silicon lenses, our metasurface lens has the advantages of thin thickness and small volume, which is conducive to the miniaturization of the terahertz detector component and provides the possibility to realize the integration with the terahertz detector.