Fiber transmission is a common mode of laser transmission, with the improvement of optical fiber preparation technology, energy transfer fiber with large core diameter and large numerical aperture is widely used in multi-mode laser transmission. Efficient laser coupling is the premise of stable transmission of optical fiber, in order to achieve laser coupling with wide band and high power density, according to the transmission transformation characteristics of mixed-mode Gaussian-like beam and the initial parameters, combined with aberration analysis, an aspherical lens coupling device between large-core four-in-one bunched fiber and single-core fiber was designed, and the maximum fiber coupling efficiency of 60.6% was obtained through mechanical installation experiments. Coupled with a self-developed laser resonance ionization time of flight mass spectrometer, the laser saturation power density corresponding to the three-color, three-step photoionization path of neodymium isotope was measured, and it was verified that the device could meet the spectral experiment requirements of specific isotope resonance excitation ionization induced by complex lasers.