Using two-step growth method and buffer layer annealing treatment, the double heterojunction structures of In_0.82Ga_0.18epilayer capped with InAs_0.6P_0.4layer were prepared on InP substrate by low pressure metal organic chemical vapor deposition (LP-MOCVD). Based on the high quality In_0.82Ga_0.18As structures, the In_0.82Ga_0.18PIN photodetector with cut-off wavelength of 2.56 μm at room temperature was fabricated by planar semiconductor technology, and the device performance was investigated in detail. The typical dark current at the reverse bias VR=10 mV and the resistance area product R₀A are 5.02 μA and 0.29 Ω·cm²at 296 K and 5.98 nA and 405.2 Ω·cm²at 116 K, respectively. The calculated peak detectivities of the In_0.82Ga_0.18As photodetector are 1.21×10¹⁰cm·Hz^1/2/W at 296 K and 4.39×10¹¹cm·Hz^1/2/W at 116 K respectively, where the quantum efficiency η=0.7 at peak wavelength is supposed. The results show that the detection performance of In_0.82Ga_0.18As prepared by two-step growth method can be improved greatly.