There are few reports on the pretreatment method based on a fractional differential algorithm for the total phosphorus content in saline soil. Therefore, the desert soil in Fukang City of XingJiang, China, was chosen as the research focus. Field hyperspectral data and surface soil down to 20 cm depth were collected between May 9–23, 2017. The Grünwald–Letnikov fractional differential algorithm was introduced to explore the pretreatment effect of the correlation between total phosphorus content and five spectral transformations. Simulation results showed that this fractional differential algorithm could elaborately describe the slight differences in spectral reflectance, and the fractional differential was close to zero on the whole. Because the sensitivity of the spectrum is controlled by the weighted order, when the order gradually increased from zero order to first order, the curve of the fractional differential slowly approached the first-order differential curve, and it slowly approximated the second-order differential curve while increasing from first order to second order. The overall trend in band number that satisfied the 0.05 significance test under five spectral transformations was an initial increase followed by decrease as the order increased. The maximum absolute values of correlation coefficients appeared at 1.2 and 1.4 order; the corresponding band was 2067 nm. This study made up for the research gap in field measurement spectra based on the fractional differential algorithm, and revealed a novel preprocessing method for field hyperspectral data.