The SGII-UP laser facility is one of the most important high power laser systems in China, and it is also one of a few inertial confinement fusion laser devices that operate all year round in the world. In order to further improve its output capacity to meet higher physical requirements, measures such as increasing the number of neodymium glasses, adopting new N41 neodymium glasses, and improving the energy configuration of xenon lamps are taken to improve the gain capacity of the main amplifier. Calculation of the new main amplifier construction model predicts that the small gain coefficient will reach 4.9%. And further modulation of the laser device shows that when the output of 10 kJ fundamental frequency energy is needed, the injection energy decreases from 5 J to 1.26 J, which supports a higher output energy and a stronger basic frequency output capability. Furthermore, it is analyzed that under different laser pulse injection conditions of 1, 5, 10 ns, the B integral is obviously reduced, which means that the near-filed quality of the beams is better. According a small-size modulation suppression is induced by nonlinear phase shift, and high-fluence laser is expected to pass before and after the improvement, which is a key prerequisite for a higher output energy. Based on these analyses, fundamental frequency output energy values with different pulse injections are calculated and an improvement from 8 kJ to 12.5 kJ output is expected under 10 ns square pulse condition. Tests show that the small signal gain coefficient of the device increases from 4.15% cm to 4.94% cm, which is consistent with simulation results, and the average gain multiple of a single beam increases from 9000 to 118000, which is an order of magnitude higher. The output verifies the fundamental frequency output capacity exceeding 12.5 kJ under 10 ns square pulse as well as a small-size modulation suppression around 0.16 mm^–1 as a result of joint action of non-linear phase shift and spatial filtering. After the significant improvement, the SGII-UP laser facility will strongly support more ambitious physical experiment targets.