The high energy, high beam quality mid-infrared lasers (3~5 μm), located in the most transparent atmospheric window band, including super-resolution molecular absorption microscopy, molecular spectroscopy, remote sensing, infrared countermeasures, military, and medical treatments, have been widely investigated for a wide variety of applications. In this research, we report on a high energy, high beam quality idler-resonant mid-infrared Optical Parametric Oscillator (OPO) based on a KTiOAsO₄ and pumped by a conventional Q-switched Nd:YAG laser. The all solid state Q-switched Nd:YAG laser wavelength at 1.064 μm (25 ns, 50 Hz, and 20 mJ) was used as a pump source, its nearly Gaussian spatial formed output was loosely focused a waist radius of ω₀ = 500 μm to the center of the crystal. A Half-Wave Plate (HWP) and a Thin-Film Polarizer (TFP) were used to control the pump energy injected into the OPO. The idler-resonant oscillator compromised of a concave input mirror (R=500 mm) with antireflection for pump and high-reflection for signal and idler outputs, and a plane output coupler with partial reflection (R=80%) for idler output, and high transmission to the pump and signal outputs. The physical length of this nearly half-symmetric OPO cavity was fixed ~ 35 mm. The compact half symmetric-stable cavity with appropriate radius curvature of the cavity mirrors provided high efficiency near- and mid-infrared generation. The KTiOAsO₄ crystal as a nonlinear parametric gain medium has high transmission in the 3~5 μm region, it was cut along its X-axis (θ=90°, φ=0°) to realize type Ⅱ Noncritical Phase Matching (NCPM) among pump, signal and idler outputs. A Ge filter is used to filter out the signal and undepleted pump beam from the idler output. We recorded the spatial form of pump beam using a conventional CCD camera, a Spiricon pyroelectric camera was also used to observe spatial forms of the signal and idler outputs. With this idler-resonant OPO, we have obtained 2.91 mJ of 1.535 μm near-infrared and 1.13 mJ of 3.468 μm mid-infrared energy outputs at the maximum pump energy of 20.2 mJ, corresponding to the slope efficiencies of 20.9% and 8.1%, respectively. In this system, although the mid infrared idler beam has a large beam divergence, when it is reflected back to the crystal by the input concave mirror with a radius of curvature of 500 mm, it is size reduced to the same size as the pump beam. This greatly increases the spatial overlapping efficiency of the pump and idler fields in a nonlinear crystal, resulting high energy, high beam quality mid-infrared output being achieved in this idler-resonant half symmetric-stable cavity. It is also worth noting that the compared to the signal-resonant OPO with the same pumping and cavity condition, in the idler-resonant OPO, idler beam with large angular dispersion allows to generate narrower spectral bandwidth and higher quality beams, it is an effective approach to improve the beam quality of the mid-infrared output radiation from OPO. We also measured the beam quality factor (M²) of the mid-infrared idler output by using the knife-edge method, resulting beam quality factors of the idler outputs were Mx2≈1.1,My2≈1.1along the horizontal and vertical directions, respectively. These values significantly indicated that the idler-resonant OPO can generate excellent beam quality mid-infrared outputs. We believe that a high energy, high beam quality mid-infrared beam will play an important role in the variety of applications. Further selecting an appropriate nonlinear crystal and optimizing the cavity design are expected to yield improved OPO performance.