By introducing an intermediate-mass into the nonlinear three-parameter isolator with an X-shaped mechanism, an improved three-parameter isolator is proposed, and its theoretical model is established. The Har? monic Balance Method is used to obtain the steady-state analytical solution of the vibration isolation system, and the correctness is verified via the fourth order Runge-Kutta method and ADMAS. By using the force transmissibility as the indicator for evaluating the isolating performance, some comparisons are carried out among the proposed isolators. Meanwhile, according to the engineering application, the time-domain displacement responses of three types of isolat? ing systems under multi-frequency steady excitation are solved, and a comparative study is carried out, respectively. The vibration power flow and maximum kinetic energy are investigated, and some typical design parameters are se? lected and analyzed. At last, from the perspective of the vibration dynamic absorber, the effects of the nonlinear connection on the vibration suppressing performance are further discussed. The calculation results show that the natural frequency of the vibration isolator can be decreased after the intermediate mass is introduced into the system, the ef? fective vibration isolation frequency band of the vibration isolation system becomes wider, and an anti-resonance fre? quency is introduced at the original resonant frequency at the same time. Compared with the traditional threeparameter isolator and the nonlinear vibration isolator with an X-shaped mechanism, the vibration isolation perfor? mance of the developed isolator at both low and high frequencies is improved accordingly. In addition, the abovementioned all design parameters exist the optimal values when the force transmissibility of the nonlinear isolator is used as an evaluating index. Besides, the vibration suppressing performance of the vibration dynamic absorber with a grounded X-shaped mechanism is enhanced.