The focal shift effect of terahertz （THz） beam focusing when using a wide-aperture refractive lens has been investigated. The deviation of focus position caused by the focal shift effect can adversely affect the imaging or measurement quality of a THz system. In this study， reference values of relative focal shifts and combinations of different lens apertures， focal lengths， and working frequencies were analyzed and discussed through theoretical calculation and finite element analysis simulation. When using the commercial lens， the actual focus was determined based on the focal shift effect to ensure the working efficiency of a terahertz system. Concerning the customized lens design， the focal shift distance was compensated in the focal length optimization according to the working frequency. These two approaches guaranteed the good performance of a THz system.