We designed a low-filling-factor and polarization-sensitive superconducting nanowire single photon detector (SNSPD), which can achieve a high absorption efficiency and counting rate simultaneously. Numerical simulations show that high absorption efficiency can be achieved by low-filling-factor SNSPDs with a silicon slot and silver reflector. The absorptance of the NbN nanowire for a transverse magnetic (TM) wave at the wavelength of 1550 nm can be 84.4% when the filling factor is only 16.5%, and the corresponding polarization extinction ratio (PER) is 562.9; the absorptance of the NbN nanowire for a transverse electric (TE) wave can be 67.0% when the filling factor is only 11.7%, and the PER is 7.4.