A high-efficiency broadband polarization converter based on metasurface is proposed in order to broaden the working bandwidth of the polarization converter in terahertz band as well as improving the polarization conversion ratio. By adding a couple of resonating branches, the original single resonating point is increased to two, dramatically increasing the working bandwidth of the polarization converter. The low co-polarized reflectance is acquired through the characteristic of high-resistance surface when the metasurface is under electro-magnetic resonance, which could enhance the overall polarization conversion rate. The simulation results illustrate that this structure is able to achieve high polarization conversion in 0.401-0.555 THz with conversion ratio above 95%. The mechanism for high-efficiency broadband polarization conversion is discussed through simulating the reflect phase and surface current distribution. The dependence of the structure on the incident angle of terahertz waves is studied. The results show that this structure is able to maintain high conversion ratio at incident angle within 0°-40°. In addition, this design has advantage of simple structure. These works can help in the design of broadband efficient metasurface polarization converter.