The model of subwavelength grating (SWG) with several dielectric layers under it is established based on the rigorous coupled-wave approach (RCWA). SWG is introduced as reflectors to the InP based long wavelength resonant cavity enhanced photodetector (RCE PD) design. Simulation demonstrates InP based SWG’s reflectivity is lager than 99.7% in the center wavelength 1.55 μm and wide reflection bandwidth (the reflectivity is higher than 99% from 1.40 μm to 1.62 μm) is achieved at the same time. Compared with pure DBR , the total number of reflector's layers is no more than 9 (4 pairs of DBR and one SWG). Thus it is an innovation of designing long wavelength RCE PD by SWG reflector (SWG-RCE PD) instead of DBR. And the results of simulation show that SWG-RCE PD’s quantum efficiency is about 90% at 1.55 μm and the product of crosstalk attenuation and quantum efficiency is more than 15 dB.