The advent of coding metasurfaces (MSs) has established a bond between electromagnetic scattering and digital control. Given that most of the current coding MSs are based on reflection-type, we investigate a novel transmission-type MS that can achieve ultra-wideband and efficient polarization conversion. A new double diagonal split ring resonator structure is designed to mainly constitute the unit. The 1-bit and 2-bit coding arrays are constructed using the proposed polarization rotation mechanism of the MS unit combined with the dimensional transformation to achieve beam focusing, beam deflection, and beam splitting. Finally, the broadband focusing characteristic of the coding MS is also demonstrated. Numerical simulation results demonstrate that the transmission coefficient of the proposed MS is above 90% in the frequency range of 0.4 to 2 THz, with a relative bandwidth of 133.3%. Asymmetric transmission phenomenon is observed in the range of 0.4 to 2 THz, with coefficients remaining above 80% in all ranges.The proposed coding MS can be used in terahertz polarization converters and unidirectional transmission devices due to its superior polarization conversion performance.