Abstract: Objective To address the challenges of high path loss and susceptibility to blockage in 5G millimeter-wave communications, as well as difficulties in antenna installation and limited capacity in ship communication environments, a 28/38 GHz dual-band four-element flexible MIMO antenna based on a photo paper substrate is designed. Methods A cross-shaped monopole antenna element with coplanar waveguide (CPW) feeding is adopted. Dual-band resonance is achieved by etching symmetrical rectangular slots in the ground plane and introducing a central slit in the radiating patch. A cross-shaped decoupling structure is integrated into the four-element MIMO system to enhance port isolation. The antenna utilizes low-cost, biodegradable photo paper as a flexible substrate, with overall dimensions of 25 mm × 40 mm × 0.27 mm, and is fabricated using a screen-printing process with copper paste. Results Measurement results demonstrate that the antenna exhibits dual-band characteristics at 28 GHz and 38 GHz, with port isolation better than -32 dB, an envelope correlation coefficient (ECC) below 0.004, diversity gain (DG) above 9.999 dB, and radiation efficiency exceeding 80%. Conclusion The proposed antenna achieves dual-band high-isolation MIMO performance within a compact structure, while also offering flexibility, lightweight properties, and environmental sustainability. It is particularly suitable for high-capacity, high-reliability millimeter-wave communication systems in complex environments such as ships, providing a new pathway for the development of green electronics and sustainable communication devices.