In this communication, a definition for the active differential reflection coefficient of the differentially-fed antenna array is proposed.  The each antenna is fed by two ports which input the equal-amplitude anti-phase signals. According to the active reflection coefficient of the conventional single-ended antenna array, the active differential reflection coefficient definition of the differential antenna array is given and analyzed. To verify the active differential reflection coefficient definition, the differential three-element microstrip patch antenna array is designed, fabricated and measured. In addition, it is derived and obtained that the active differential reflection coefficient of the differential port m is a half of the active reflection coefficient sum for the single-ended port m+ and m-. The active differential reflection coefficient definition will give a port match design guidance for the differential antenna array.