A polarizer is used to eliminate undesired polarization states and maintain an orthogonal one. The polarizer we proposed is designed on a silicon nitride (Si3N4) on insulator platform to achieve low-loss operation at the 850 nm wavelength region. Compared with conventional polarizer structures, chirped subwavelength gratings (SWG) are introduced in the proposed device's main body to extend the wavelength band of the leakage mode, i.e., TM polarization state. Owing to the broadband nature of leakage mode, the operating bandwidth, which is defined as the wavelength region with extinction ratio (ER) higher than 20 dB, is increased dramatically. The simulation results show that the TE polarization state passes through the proposed polarizer with a high ER=46.24dB and a low insertion loss (IL)=0.13dB at 850 nm. A bandwidth broader than 171 nm is achieved with ER>20dB and IL<1dB over the wavelength region from 775 to 946 nm.