A 13 J direct-liquid-cooled solid-state disk laser with an unstable cavity is developed and demonstrated in this paper. The output energy of the resonant cavity is first analyzed according to the gain level in a stable cavity. At the optimum gain level, a magnification of 1.3 in the unstable cavity is achieved. Experimentally, a concave-convex mirror is used as the cavity mirror. At a magnification of 1.3, a repetition frequency of 100 Hz, and a pulse width of 350 µs, a single-pulse energy output of 13.2 J is obtained, corresponding to an optical-optical efficiency of 22% and a slope efficiency of 27.6%. The x-axis beam quality factor β is 4.7, and the y-axis beam quality factorβ is 16.6.