Abstract: Quasi-zero stiffness (QZS) isolators have the feature with high static and low dynamic stiffness and widely been focused in the field of the vibration isolation at low frequency. Isolators with nonlinear QZS can’t be used for the low frequency vibration isolation under variable mass loads because the increased dynamic stiffness caused by mismatched mass load raises the initial frequency of vibration isolation and increases the magnitude of transmissibility. In order to solve this problem and improve the performance of vibration isolation at low frequencies, an isolator has been designed by using tension springs and oblique bars. Two QZS conditions including parameters have been derived on the basis of zero values of the stiffness and its second-order derivative at the static equilibrium position. According to the parameter conditions, constant QZS, constant zero stiffness or constant force, and nonlinear QZS can be realized as any values desired. The vibration isolation method with multi-stage consecutive constant QZS is furtherly proposed based on the constant QZS. The displacement transmissibility has been derived and calculated by employing the harmonic balance method and the incremental harmonic balance method, and shows the same result with each other. Prototypes with single-stage and multi-stage constant QZS have been fabricated to experimentally study the two vibration isolation mechanisms of variable mass loads. The first mechanism is the single-stage constant QZS to successfully isolate vibrations with the small magnitude of variable mass loads, such as 10% variation of the designed mass load. The second mechanism is the multi-stage consecutive constant QZS, which can be applied to variable mass loads with the large magnitude, such as the mass load is 32% variation of the designed value or larger extent. The two mechanisms of the proposed QZS isolators provide practicable approaches for the vibration isolation at low frequencies under variable mass loads.