A novel two-element UWB-MIMO ground antenna is designed by using the theory of characteristic modes. The proposed antenna has a simple and compact coplanar structure, which consists of a rectangular metal ground, a four-stage stepped patch, a double L-shaped patch with a corner cut and a rectangular substrate. By analyzing the most relevant characteristic modes of the metal ground in UWB, the expected characteristic modes are excited by the capacitive coupling elements and the hybrid loading of the capacitive and inductive coupling elements, so as to reduce the size, broaden the bandwidth and improve the isolation. The simulated and measured results show that the proposed antenna obtains ultra-wide impedance bandwidths (2.7-12.6 GHz for Port 1 and 3.0-11.0 GHz for Port 2). Furthermore, the proposed antenna also achieves high gains (3.1-7.3 dBi for Port 1 and 2.7-5.8 dBi for Port 2), stable radiation patterns and good diversity characteristics (the minimum isolation > 16 dB, the envelope correlation coefficient < 0.01, the channel capacity loss < 0.08 bps/Hz, and the total active reflection coefficient < -4.1 dB, etc.) in the whole impedance bandwidth. The research results can provide a useful reference for the design of UWB-MIMO ground antennas based on the theory of characteristic modes.