By optimizing the parameter design, this paper prepared a 3×1 photonic lantern with excellent performance. It was fused with a multimode fiber with a core diameter of 30 μm. The stable and controllable quasi-single-mode output is achieved by spatial mode adaptive control and the traditional design principle of photonic lantern mode matching is broken through. Moreover, the optional range of the core diameter at the photonic lantern multimode fiber end is expanded. The stochastic parallel gradient descent (SPGD) algorithm is used to actively control the phase modulator at the input end. When the evaluation function is selected as the barrel power, the fundamental mode proportion of the output beams at the multi-mode fiber end is higher than 85%, and the quality factor M² of the beams is stably below 1.18, which provides a possibility to solve the problem of mode instability in large-mode-field fiber laser. When the evaluation function is selected as the mode content of a single high-order mode, the corresponding high-order mode content at the output end exceeds 95%.