Abstract: The mechanical cryocooler is a critical component for providing the low-temperature environment essential for space-based infrared payloads. The vibrations generated during its operation, primarily from compressor reciprocation, constitute a significant disturbance that can degrade infrared detector performance through image jitter and reduced pointing accuracy. To suppress vibration transmission, this paper presents a passive vibration isolation device based on a monolithic flexure spring structure. The design eliminates moving contact points, thereby avoiding friction, wear, and particulate generation—key advantages for long-duration space missions. Guided by classical vibration isolation theory, the system parameters were optimized to minimize vibration transmissibility, with the natural frequency set significantly below the compressor's fundamental excitation frequency. A parametric finite element method was employed to determine the flexure spring's structural dimensions. Critical geometric parameters, including beam thickness, length, and curvature, were defined as variables and iteratively optimized through finite element analysis. This process balanced the requirement for low stiffness—necessary for effective high-frequency isolation—against the need for sufficient mechanical strength to withstand launch and operational loads. Experimental validation was conducted using a test setup equipped with tri-axial accelerometers. Vibration levels were measured at the cryocooler mounting point and the isolated base across a frequency spectrum encompassing the compressor's fundamental frequency and its harmonics. The results demonstrate significant simultaneous vibration attenuation along all three translational axes （x, y and z）. Specifically, the device achieved an axial （z-direction） vibration isolation efficiency of 90% at the compressor’s fundamental frequency, corresponding to a transmissibility of 0.1. Substantial suppression was also observed in the lateral directions. These findings validate the design methodology and confirm the device's efficacy in providing a stable mechanical environment, thereby enhancing the performance and reliability of sensitive space-based infrared detection systems.