Abstract A quasi-zero-stiffness (QZS) isolator can perform a good function of low-frequency vibration isolation, but its designed ultra-low dynamic stiffness could be notably affected by parametric errors, which usually occur in the manufacturing and assembling process. In this paper, the effects of design parameter errors on the performance of a torsion QZS vibration isolator are studied, and several useful guidelines for the manufacture and assembly of the QZS isolator are achieved. An approximate relationship between a single error and the jump-down frequency is obtained, and then the influence of a combination of multiple errors is investigated numerically. The results indicate that, as key parts of the QZS isolator, the universal wheel and cam should avoid positive manufacturing tolerances, while the shaft connector has to keep away from negative tolerance, and an assembly error of the disk spring stack should be mostly avoided as much as possible. Additionally, the degradations on the performance caused by single errors can be eliminated by a combination of multiple errors, due to a mutual counteraction of these influences. Most importantly, the QZS vibration isolation system still outperforms its linear counterpart, even though it has parametric errors.