A multi-band multi-antenna system and a communication device thereof are provided. The multi-band multi-antenna system includes at least one ground, two antenna units, a coupling conductor line and a grounding conductor line. Both of the two antenna units have at least one conductor portion, a low-pass filtering portion and an extending conductor portion. Each antenna unit generates at least one higher and lower operating bands. The low-pass filtering portion is electrically coupled between the conductor portion and the extending conductor portion, and effectively decreases dependent relationship between the higher and lower operating bands. The coupling conductor line is disposed nearby the two antenna units and has a first coupling portion and a second coupling portion. The grounding conductor line is disposed between the two antenna units and connected to the ground.