An approach of designing polarization-insensitive, broadband spectrum, high-reflectivity mirror based on blocky structure with subwavelength grating is proposed. The reflectance properties of subwavelength grating are determined by structure parameters of grating such as period and duty cycle. Optimizing the period and duty cycle can get a higher reflectivity at broader spectrum. The reflectivities of grating with different periods are calculated by rigorous coupled wave analysis. A set is formed with grating periods of reflectivities over 99%. The size of data set is reduced using an iterative optimization algorithm, which can quickly determine the optimal grating period and the corresponding duty cycle. The simulation results show that two-dimension blocky structure with subwavelength grating mirror fabricated with standard silicon-on-insulator (SOI) wafer can keep the reflectivity over 99% at a spectrum range of 182 nm. Meanwhile the reflectance properties of grating is independent of polarization of vertical incident light. But it can′t keep the reflectivity over 99% at a wavelength range from 1439 nm to 1621 nm anymore as the incident angle up to 7°. So the can be applied on vertical cavity surface emitting lasers (VCSEL) to achieve velocity measurement of laser Doppler or two-dimension network structure of optical buffer. Furthermore, numerical simulations show that the working process of two-dimension blocky structure with subwavelength grating mirror has some tolerance of period and duty cycle.