Carrier Phase-Shifted SPWM (CPS-SPWM) technique has been widely used in the switching modulation strategy of multilevel converters. Under the same conditions, unipolar dual frequency CPS-SPWM can obtain good bandwidth performance compared with bipolar CPS-SPWM. The transmission bandwidth is especially important in the harmonic control of Active Power Filter (APF) because it is the key performance index of multilevel converter. The principle of unipolar dual frequency CPS-SPWM is described and the output voltage formula of H-bridge cascaded converter is derived by double Fourier. Through the definition of noise margin, the order of Bessel function is determined, then the transmission bandwidth of the unipolar dual frequency CPS-SPWM is studied and its quantitative expression is got. Finally, the simulation and experimental results verify the accuracy and rationality of the transmission bandwidth of the proposed unipolar dual frequency CPS-SPWM. This work is supported by National High-tech R & D Program of China (863 Program) (No. 2015AA050104).