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We experimentally demonstrate a stable and compact high pulse energy mid-infrared (MIR) 6.45 μm laser, which is produced by an optical parametric oscillator (OPO) based on the non-oxide BaGa4Se7 (BGSe) nonlinear crystal pumped by 1064 nm Nd:YAG laser oscillator. With optimizing the parameters of the OPO system, a record high idler energy of 6.76 mJ at 6.45 μm was obtained with 18.85 ns of pulse duration (FWHM) at the repetition rate of 10 Hz under 93.5 mJ of pump energy, corresponding to an optical-to-optical conversion efficiency up to 7.23% from 1064 nm to 6.45 μm. The experimental results were in close approximation with the theoretical calculations. To date, it is both the highest MIR pulse energy and the highest conversion efficiency at 6.45 μm for any 1 μm pumped OPOs. Further, excellent output energy stability is demonstrated, with an energy fluctuation of less than ±0.5% over 165 min. Such a laser is interesting for both scientific and medical applications, including, for instance, for trace gas analysis and tissue ablation.