An active distributed antenna system and a relay unit thereof are disclosed. The relay unit includes: at least two access units, first combiners and optical units; a number of first radio frequency switches, second radio frequency switches and second power dividers corresponding to the first combiners respectively; a second combiner and a first power divider; wherein each of the first combiners is coupled at one end to at least one of the access units, and the other end is coupled to the movable terminal of the first radio frequency switch; one end of the second combiner is coupled to the first stationary terminal of each of the first radio frequency switches respectively, and the other end is couple to the first power divider; the other end of the first power divider is coupled to the first stationary terminal of each of the second radio frequency switches respectively; the second stationary terminal of each second radio frequency switch is coupled to the second stationary terminal of a corresponding first radio frequency switch, and the movable terminal of each second radio frequency switch is coupled to one second power divider; and the other end of each second power divider is coupled to at least one optical unit. The SISO mode and the MIMO mode are compatible in the relay unit according to the present invention, so the network construction is at a low cost.