In order to implement a solar direct pumping slab high power laser, a linear uniform high-power density pump source is studied. In this paper, we propose a design method of a high-power density uniform linear light source by combining the first-order concentrating system with seven confocal ellipsoids to form a composite ellipsoid cavity. The equal radiation flux segmentation of the circular first focal spot is realized by each ellipsoid. The mirror imaging characteristics do not significantly decrease the peak power density. After decomposition, the mirror spot forms a uniform linear light source at the second point of focus. The mathematical model of equal radiation flux is given by coordinate transform, and the rotation and translation parameters of each ellipsoid are solved by the annealing algorithm. The first-order system is composed of a Fresnel lens with a radius of 30 mm, a focal length of 70 mm and a single ellipsoidal cavity with a of 3.4 mm, cof 3.15 mm, The second-order composite ellipsoidal cavity concentrating system is attached. The line source is realized with effective length of 12 mm, the peak power density of 1.09 × 10⁶ W/m², and the uniformity of 95.46 %. Compared with the contribution of each ellipsoid parameter to the uniformity, the uniformity effect is significantly improved when the rotation parameter θ of the middle ellipsoid is 1.4°. The change of the edge ellipsoid parameter Δ has a significant influence on the effective length of the linear light source, and its optimal value is 0.53 mm.