This paper proposes a broadband, low-profile, dual-circularly polarized (CP) active phased array modular subarray. The subarray elements adopt dual-CP stacked microstrip patch antennas to expand the bandwidth. Each subarray has a complete structure and function, which can be used independently or extended to meet the requirements of different application scenarios. The impact of asymmetric plane is counteracted by a mirror-feed technique, which improves the consistency of scanning gain and axial ratio (AR) at both positive and negative angles. Meanwhile, the subarray can simultaneously form two beams of left-handed circular polarization (LHCP) and right-handed circular polarization (RHCP) for scanning. Finally, a modular subarray prototype with an 8 × 8 array scale was simulated, fabricated, and measured. The simulation results show that the scanning gain decreases by less than 2.19 dB at ±45° and 5.4 dB at ±60° at the frequency of 12.1 GHz. Moreover, within the frequency range of 10.23–12.77 GHz (22.08%), the active voltage standing wave ratios (VSWR) of LHCP and RHCP in the azimuth and elevation dimensions of the subarray are below 2, and the AR in the beam scanning range of ±45° and ±60° are less than 3 and 6 dB, respectively.