By combining the quasi-zero stiffness structure （QZS） with an X-shaped isolator， a new passive X-shaped quasi-zero stiffness isolator （X-QZS） is designed. By replacing the linear spring of the X-shaped isolator with the QZS， excellent performance in ultra-low frequency band is obtained. Harmonic balance method and arc length method are used to solve the motion equations established by the Lagrange equations in order to obtain approximate analytical solutions ，which are then verified by numerical method. Steady-state responses to harmonic displacement excitation with different structural parameters are compared. The vibration isolation performance are analyzed. The results demonstrate that X-QZS can achieve better isolation than QZS in the ultra-low frequency range， with lower resonance frequency and wider vibration isolation band.