We propose a bi-layer transmissive metasurface to obtain linear-to-circular polarization conversion in a wideband. The unit cell of each metasurface layer, which has identical configuration, consists of a Jerusalem-cross-like resonator and a metal strip. A universal equivalent circuit model is employed to guide the design of the polarizer. By analyzing the circuit parameters of the equivalent circuit, four metal strips are symmetrically inserted in each unit cell to broaden the bandwidth of linear-to-circular polarization conversion. Numerical and experimental results show that the polarizer can convert a linearly polarized wave into a circularly polarized wave in a wideband from 6.1 GHz to 12.6 GHz. Compared to the reported designs, the proposed polarization converter has advantages of ultrathin thickness and ultrawideband, and hence can be used in many applications, such as antennas and remote sensors.