Abstract: To address the degradation of system performance caused by mutual coupling among antenna elements in multi-input multi-output (MIMO) systems for 5G wireless communications, an eight-element MIMO antenna based improved parallel-combined CSRR decoupling structure design tailored for WiFi applications is proposed. The mutual coupling effect among the MIMO antenna units in a 5G wireless communication system leads to a degradation of the system performance. To address this issue, an improved parallel combined CSRR (complementary split-ring resonator) decoupling structure for an eight-element MIMO antenna design tailored for WiFi application scenarios is proposed. The antenna employs a decoupling technique based on CSRRs, achieving an optimal balance among compact structure, wideband operation, and high isolation performance. Experimental results demonstrate that the antenna achieves a −10 dB impedance bandwidth of 19%, with isolation levels of S₂₁ > 16.7 dB and S₈₁ > 13.7 dB within the 5.50 GHz to 6.66 GHz operating band; and the antenna system achieves a radiation efficiency of 50% to 86%, a peak gain of 3.28 dBi, an envelope correlation coefficient (ECC) below 0.1, and a diversity gain exceeding 9.91 dB, meeting the requirements for 5G mobile terminal applications. By analyzing the impact of CSRR geometric configurations and spatial arrangements on system performance, this study reveals that the parallel-combined of CSRR structure enhances the isolation through a triple synergistic mechanism comprising resonance band-stop effect, suppression of surface waves, and near-field electric field reconstruction. The research findings provide theoretical insights and technical pathways for the decoupling design of high-performance MIMO antennas.