In this paper, a nonlinear crystal corner-cutting method is proposed to change the laser mode distribution on the output face of the PPMgLN crystal by cutting off a corner of the output face of the PPMgLN crystal in the resonance cavity, so as to realize the stable fundamental mode output of the 561 nm yellow laser. Using the finite-difference eigenmode method, the laser mode distributions at the output end face of various corner-cutting schemes were simulated to discuss the principle of their output fundamental mode, and the change curves of the output power of each scheme with the pump power were measured experimentally. The results show that when the pumping power is 4.8 W, the highest fundamental mode output of 121 mW is realized, with an optical-to-optical conversion efficiency of 2.6%, a central wavelength of 560.95 nm and a full width at half maximum of 0.92 nm, and an instability of less than 1.91% in 3 h. With a smaller structure size compared with the reported experimental results, this experimental result will provide an important reference for compact fundamental mode lasers.