A diode pumped passively Q-switched 1123 nm laser is reported in this paper; and a mixed crystal Nd:LuYAG is selected as the gain medium. A large number of excellent properties from Nd:YAG are obtained, and the mixed crystal Nd:LuYAG has been used widely in all-solid-state lasers. Besides, compared with Nd:YAG, the Nd:LuYAG has some other wonderful advantages. For example, both the absorption bands and the fluorescence line are broadened, resulting from the crystal strong inhomogeneity. Their wide absorption makes the Nd:LuYAG lasers' pump source not rigorous in their temperature control. And the broadened fluorescence line can generally improve the laser performance in Q-switched regimes. In this paper, a concave-plane configuration cavity with its length as long as 35 mm is designed to achieve high-efficiency laser output. The rear mirror is a concave mirror with a curvature radius of 300 mm, and the output coupler is a flat mirror with a transmission of 2% at 1123 nm, 5% at 1112 nm, 4% at 1116 nm, and has high transmissions at 1064, 1319 and 1444 nm respectively. A Cr4+:YAG crystal, with its initial transmission of 97%, is used as the saturable absorber. In the continuous wave operation, the maximum average output power can reach 2.77 W, with the corresponding optical-to-optical conversion efficiency of 29.53%. In Q-switched operation, the maximum average output power is 0.94 W at 9.38 W absorbed pump power. The repetition rate is 9.40 kHz, with the corresponding single pulse energy being 100 J. The high single-pulse energy explains that the Nd:LuYAG mixed crystal is better than Nd:YAG in high energy storage. Only one wavelength can be observed in our experiment. The center wavelength is 1122.7 nm and the line width is 0.03 nm. To the best of our knowledge, this is the first time to report the Nd:LuYAG mixed crystal laser emitting at 1123 nm.