In this paper, a novel broadband circularly polarized transmitarray antenna (CPTA) enabled by axial-ratio-improved receiver-transmitter metasurface loaded with parasitic patches is proposed. Split-ring-shaped parasitic patch is utilized to generate an additional resonant mode and significantly broaden the 3-dB axial ratio (AR) bandwidth of proposed receiver/transmitter patches from 6.64% to 15.61%. By cascading the receiver and transmitter with the same polarization and then rotating the cell, Pancharatnam-Berry phase can be exploited for providing a 2π phase shift. As verification, a CPTA prototype integrated with a self-made circularly polarized patch antenna is designed, fabricated, and measured. Experimental results show that the proposed CPTA obtains a 3-dB AR bandwidth of 27.1% from 12.1 to 15.9 GHz and an impedance bandwidth of 20.6% from 12.5 to 15.2 GHz. Additionally, it has a flat gain with a 3-dB gain bandwidth of 18.8% from 12.5 to 15.1 GHz, and a maximum gain of 25.6 dBi at 13.1 GHz is achieved. With the advantages of simple design, wide AR bandwidth, and flat gain performance, the proposed CPTA presents great potential applications in wireless systems.