The performance of a photonic functional device in bulk CMOS has been limited by the high propagation loss in polysilicon strip waveguide. Based on the zero-process-change methodology, we successfully reduce the propagation loss of polysilicon waveguide from 112 dB/cm to only 38 dB/cm by solely engineering the waveguide geometry for the first time. Low propagation loss is attributed to a significantly reduced optical overlap factor of 0.09 to bulk polysilicon using subwavelength grating (SWG) waveguide design. These findings prompt us to demonstrate a narrowband SWG-based cladding-modulated Bragg reflector in bulk CMOS, which provides a full-width at half maximum (FWHM) of 1.63 nm, an extinction ratio of 24.5 dB, and a reduced temperature sensitivity of 27.3 pm/°C. Further reducing the FWHM to 0.848 nm is also achieved by decreasing the grating coupling strength. We believe the achievements made in this work validate a promising design path towards practical photonic-electronic applications in bulk CMOS.