An unstable resonator can obtain high extraction efficiency of light as well as high beam quality under the condition of the large Fresnel number. We design a telescopic off-axis metastable-unstable hybrid resonator with conduction-cooled end-pumped structure which can be used as the Nd∶YAG slab gain medium, and its output coupling mirror is a variable reflectivity mirror. The resonator in the width direction of the hybrid resonator (the x direction of the slab gain medium) is an unstable resonator, and the resonator in the thickness direction (the y direction of the slab gain medium) is a metastable resonator. Through the theoretical analysis, an output power of 4428.7 W of this metastable-unstable hybrid resonator structure and an optical-optical efficiency of 0.4429 can be obtained when the pump power is 10 kW.