In the current 5G era, antennas with different functions are being assembled together to save tower space and facilitate subsequent maintenance. However, the assembly would deteriorate the antennas’ radiation pattern and isolation. In this letter, we propose an effective method for this typical application scenario. Considering the example of a tri-band antenna, a double-strip structure is first set around the arms of the low-band element to enhance the mutual isolation between the low- and middle-band elements. Then, a metamaterial surface is inserted between the low-/middle- and high-band elements to reduce the distortion of the radiation pattern. An embodiment is fabricated to verify the design. Simulation and test results show that the proposed combined decoupling method is effective for a hybrid antenna system.