Mid-infrared (MIR) nonlinear optical crystals of LiInSe2 (LISe) were grown by a modified Bridgman technique on a (001)-seed. A 7-12 μm widely tunable picosecond (ps) MIR optical parametric amplifier (OPA) based on a LISe crystal was demonstrated for the first time, to the best of our knowledge. The MIR OPA was pumped by a 30 ps 1064 nm Nd:YAG laser and injected by a KTiOPO4 (KTP)-based widely tunable near-infrared seed. The idler operating at 7.5 μm with the highest pulse energy of 170 μJ was obtained under a pump energy of 14 mJ. The corresponding energy conversion efficiency is ∼1.21%, and the photon conversion efficiency is 8.6%. The output energies were measured to be ∼121 μJ at 7 μm and ∼21 μJ at 12 μm.