Abstract: To investigate the spectral properties of Laguerre-Gaussian ultrashort pulsed lasers transmission in a gradient refractive index fiber, the generalized Huygens-Fresnel principle was employed to deduce the analytical expression of the Laguerre-Gaussian ultrashort pulsed lasers transmission in a gradient refractive index fiber, and the reliance of the spectral modulation function on topological charge, radial index, and fiber transmission distance was analyzed. The research reveals that when the Laguerre-Gaussian ultrashort pulsed lasers is transmitted in a gradient refractive index fiber, the spatial distribution of the spectral modulation function and the spectral intensity undergoes periodic focusing and diffusion, yet the temporal-spatial coupling effect leads to their non-symmetrical distribution. For beams with different central wavelengths, the longer the wavelength, the higher the second peak of the spectrum after being modulated by the spectral modulation function. In addition, when the duration of the Laguerre-Gaussian ultrashort pulse laser is 3 fs and the transmission distance is certain, the spectrum splits into two peaks with equal peak heights at the off-axis position of 8.99 μm, and then the phenomenon of spectral switching can be observed, and the spectral shift rapidly jumps from the red-shift to the blueshift when the off-axis position crosses the point. This study offers theoretical support for the spectral variations of the Laguerre-Gaussian ultrashort pulsed lasers transmission in a gradient refractive index fiber and holds potential applications in information coding and transmission.