In this paper, we report an incoherent space laser beam combiner for rectangular beam combination with 18 fiber-transmitted semiconductor lasers at 972 nm wavelength. The coupled thermo-mechanical properties of all the optical lenses in the combiner were analyzed to evaluate the long-term reliability of the combiner subjected to beyond 10 kW high-power laser irradiation. An experiment-based numerical model was developed on the basis of the multi-beam laser volumetric heat source, and the performance of lenses irradiated by 10 kW combined laser for 1000 s was studied with finite element method (FEM). The maximum temperature among all lenses was 427.27 K which is much lower than the softening point temperature of fused silica material made of lenses. And the 0.1 aperture number corresponding to the maximum thermal deformation of 4.53 µm is much smaller than the conventional optical lens processing tolerance. The maximal stress of 12.73 MPa is far less than the yield stress of 4.5 GPa for the material. The manufactured laser beam combiner realized a highest output power of 10.641 kW at a power combination efficiency of 98.5%, a combination length of 200 mm and a rectangular focal beam spot of 30 mm × 10 mm. Our method provides a valuable choice for realizing fast and flexible laser surface heat treatment. © 2022 Elsevier GmbH