A space-transmission high-power near-infrared semiconductor laser beam with a rectangular laser spot is one of the key tools to improve the efficiency and quality of laser surface heat treatment. However, this kind of laser is difficult to apply to the surface heat treatment of flexible lasers inside workpieces. This is because the volume of the lasers increases greatly with increasing laser power and is affected by the space transmission of the laser beam. The spot of the ultrahigh-power laser beam from a commercial high-power fiber laser or a fiber-transmitted semiconductor laser is circular, which makes it difficult to control the spot overlap rate during the laser surface heat treatment process. It is difficult to change the ultrahigh-power circular laser spot into a rectangular spot through beam shaping technology. Laser incoherent spatial combining based on multifiber transmission is an effective method to reduce the risk of high-power laser transmission in a single fiber and realize the flexible transmission of high-power lasers. It has quickly become a research hotspot in the field of ultrahigh power laser systems. To solve the spot overlap rate control problem of ultrahigh power lasers transmitted by fibers in flexible laser surface heat treatment, a design scheme of arranging 18 semiconductor laser beams at 972 nm transmitted by fibers according to a “staggered rectangle” and implementing space incoherent beam combination was proposed in this paper. Based on this, a set of 10 kW rectangular spot laser beam combiners was developed. The optical elements in the combiner are all optical lenses made of fused silica glass. It is widely known that the accumulation of heat generated by long-term ultrahigh-power laser beam irradiation will produce serious thermal effects inside the optical lens, resulting in reduced beam quality and even irreversible damage inside the optical lens, which will seriously affect the safety and reliability of the combiner for long-term operation. However, the structural shielding of the combiner often makes the thermodynamic properties of the optical lenses difficult to directly detect and evaluate with experimental methods. With the rapid development of computer technology and calculation methods, the establishment of temperature field models based on finite element analysis has become a simulation method widely used in the reliability analysis of laser irradiation optical components. At present, most thermodynamic finite element analysis studies on laser irradiation optical elements simplify the laser beam to an area heat source while ignoring the volume absorption of the laser beam by the optical element. However, the volume absorption of the laser beam by the optical lens itself is already one of the main factors affecting its thermodynamic properties with the continuous increase in laser incoherent space combining power. There is no report on the thermodynamic finite element analysis of multiple ultrahigh power laser beams transmitted through optical lenses under the premise that the laser beam is used as a volume heat source. To solve the above problems, the finite element thermodynamic model of the optical lens was established based on the mathematical model of the whole heat source of the 18 laser beams. The thermodynamic properties of all optical lenses under the condition of being irradiated by 18 laser beams participating in the combination for 1 000 s are simulated and analyzed using this model. The research results show that the maximum temperature, maximum thermal deformation and maximum equivalent thermal stress of the optical lens in the combiner stabilized after the 800 s time node. The simulated values of the maximum core temperature and the maximum equivalent thermal stress were 427.27 K and 12.68 MPa, respectively, which were significantly lower than the softening point temperature and thermal damage threshold of fused silica glass used to manufacture optical lenses. The maximum aperture of 0.1 corresponding to the simulated maximum thermal deformation of 4.53 μm was much smaller than the conventional machining tolerance of 2.0. The highest temperature on the exit surface of the window lens was measured during the laser beam combining time of 1 000 s. Both the experimental value and the simulated value of the highest temperature showed good consistency with the laser beam combining time. This shows that the established finite element thermal analysis model has good accuracy. The maximum combined power of 10.64 kW for the combined laser with a rectangular spot was measured when it was continuously operated for 1 000 s. The power instability of less than ±1.2% further experimentally verified the safety and reliability of the combiner under long-term operation.