Aiming at the problems of large size, poor port isolation and weak anti-interference ability of multi-band multiple-input multiple-output (MIMO) antennas, a triband MIMO antenna which can be applied to 5G n78/n79 and WiFi 6E bands is designed. The antenna is composed of a slotted diamond patch and a trapezoidal floor, and combines the innovative design of a semi-circular structure and a symmetrical inverted L-shaped branch. This design not only realizes the required triple-frequency characteristics, but also effectively controls the size of the antenna to adapt to more compact application requirements. The antenna is fed by coplanar waveguide (CPW), which has the advantage of easy integration with other microwave circuits. By placing the unit antenna orthogonally and without isolating branches, the port isolation of the MIMO antenna in the required frequency band is greater than 25 dB. The simulated antenna is processed and tested. The measured results show that when the return loss is less than-10 dB, the impedance bandwidth of the antenna is 3.28~3.67, 4.63~5.01 and 5.67~7.65 GHz, which is suitable for 5G n78/n79, WiFi 6E band. The maximum measured gain can reach 4.7 dB, the envelope correlation coefficient (ECC) is less than 0.001, and the diversity gain (DG) is greater than 9.999 9. The diversity performance is good. The measured results are highly consistent with the simulation results, which verifies the effectiveness and accuracy of the design. Considering the size advantage, good isolation and excellent diversity performance of the antenna, the triple-band MIMO antenna proposed in this paper shows great application prospects in 5G and WiFi 6E communication systems, which can meet the growing demand of future wireless communication and promote the development and innovation of wireless technology.