In this work, a broadband tunable reflective polarization convertor using a single layer complementary-cross-shaped (CCS) graphene metasurface was proposed and investigated numerically in terahertz (THz) region. The electric property of CCS graphene could be flexibly tuned through the Fermi energy level (E_f). When E_f = 0.9 eV, the proposed graphene metasurface can realize a relative high polarization conversion ratio (PCR) of over 80% from 2.15 THz to 4 THz with a relative bandwidth of 60.16%. Furthermore, the PCR peak can reach up to as high as 99%, 99.5% and 91.5% at 2.4 THz, 3.4 THz and 3.96 THz, respectively. The physical origins of this polarization conversion properties are turned out to be supported by field distributions and Fabry-Perot interference theory. Furthermore, the broadband polarization conversion properties will be regulated by varying the Fermi energy level.