The double-helix chiral fiber grating is a polarization and wavelength selective, which is very potential for applications in the field of optical fiber sensing. However, the spin pitch of the reported chiral fiber grating with circularly polarizing property is only tens of micrometers. The high insertion loss and low mechanical strength limit its application scope. The polarization filtering properties of the normally used coated fiber, whose refractive index of the jacket is higher than that of the cladding, are studied using the complex coupled mode equations. It is revealed that not only the mechanical strength increases in the coated fiber, but also the spin pitch increases to hundreds of micrometers. For this reason, loss can be reduced to meet the transmission requirement of a medium length of fiber. Influence of the parameters of the double- helix circularly polarizing fiber on the polarization filtering properties is analyzed. It is shown that increasing the coupling coefficient of the core mode and the cladding leaky mode is an effective way to improve the polarization filtering performance of the fiber. The results are instructive for the research and developing of the double-helix circularly polarizing fibers.