To reduce the transmission risk of high-power laser in a single fiber, a 19×1 incoherent space laser beam combiner with circular spot based on multiple optical fiber transmission was presented. The coaxial close arrangement based on “ inner and outer concentric circles + center” is applied in the optical design of the combiner, and the lenses parameters of the combiner are obtained by ray tracing method. Simultaneously, to evaluate the long-term operation reliability of the 19 × 1 laser beam combiner accurately, the coupled thermo-mechanical characteristics of all optics in the beam combiner is analyzed, to clarify the distributions and evolution laws of temperature field and thermal stress field of optics by using the finite element method ( FEM). The multi-beam laser volumetric heat source model is established based on the overall heat source distribution of 19 laser beams synchronously transmitted optical lenses. Then, the temperature field, thermal displacement and stress of optics are simulated. Results show under the irradiation of 10 kW laser for 30 minutes, the temperature of all optical lenses in the beam combiner with the maximum of 381. 11 K is far lower than the softening temperature of 1 900 K of the fused silica material. The thermal deformation with the maximum corresponding aperture number of 0. 07 is within the design tolerance, and stress with the maximum of 14. 02 MPa is less than the yield stress of 4. 5 GPa of the fused silica. In addition, the total laser power output by the beam combiner can reach 10. 43 kW . The study provides an effective method reference for evaluating the long-term operation reliability of other high-power laser source.