A 12.5 μm long wavelength infrared detector based on InAs/GaSb Type-II superlattice was presented in this work. Superlattice materials were grown on GaSb substrates using MBE technology. Absorber structure for long wavelength detector was designed to be 15ML(InAs)/7ML(GaSb). The detector used a PBIN multiple heterostructures to decrease the dark current. The dark current I-V curve, responsivity spectra and blackbody current responsivity were measured at 77 K. At this temperature, RmaxA product was 2.5 Ωcm2 for the device with a photo sensitive area of 100 μm× 100μm. At zero bias, a current responsivity of 1.29 A/W was measured for the detector, which correspond to a blackbody detectivity of 2.1×109 cmHz1/2/W. Quantum efficiency at 11 μm was measured to be 14.3%. Dark current characteristics were simulated with four kinds of probable transport mechanisms. The results showed that the dominated dark current of the detector is Generation-Recombination current.