InGaAs photodetector is widely used in SWIR detection. Bi incorporation into InGaAs can reduce the bandgap， extending the detection wavelength. By controlling of the In and Bi compositions， the detection wavelength could be extended to over 3 μm from In_yGa_1-yAs_1-xBi_x， lattice-matched to InP. An In_0.394Ga_0.606As_0.913Bi_0.087 p-i-n photodetector is designed and its performance is numerically investigated. Dark currents and responsivity spectra are calculated with different temperatures， absorption layer thicknesses and doping concentrations. A 50% cutoff wavelength of 3 μm is achieved. The proposed structure provides a feasible way to fabricate InGaAsBi based SWIR detector with longer detection wavelength.