Many sandy soil slopes easily deform during or immediately after rainfall infiltration, which causes soil erosion. A novel double polymer used as a chemical sand-fixing agent (DPCM) based on water-soluble polymers of carboxymethyl cellulose and polyacrylamide was studied in this paper. The resistance to water erosion of sandy soil slopes with a DPCM stabilization layer has been studied by the physical modelling of rainfall. Nine laboratory rainfall erosion tests were carried out to establish a mathematical model to calculate the soil loss on sandy soil slopes. The experimental treatments included DPCM to sand mass ratios of 0:0, 1:2 and 1:3, slope gradients of 30°, 40° and 50°, rainfall intensities of 150, 200 and 250 mm/h, and elapsed times of 10, 20 and 30 min. The results showed that the average water erosion moduli in a mixture of DPCM and sandy soil with mass ratios of 1:2 and 1:3 were 5.71 g/m²/min and 3.13 g/m²/min, respectively, which were significantly lower than the average water erosion modulus of 14.28 g/m²/min on the bare slope. The water erosion modulus of the slope treated by DPCM was 60–78% lower than that of the untreated slope, and the infiltration rate and total runoff yield from the covered plots were 42.9–53.6% and 56.1–80.2% lower than those from the controls, respectively. The sensitivities of the DPCM to the sand mass ratio, rainfall intensity, slope gradient, and elapsed time to infiltration and water erosion were 1.26, 0.70, 0.53, and 0.32 and 11.15, 1.72, 1.48, and 0.73, respectively. Finally, combined with rainfall simulation testing results, an erosion calculation model on sandy soil slopes composed of rainfall, infiltration rate and slope characteristics was obtained. A comparison analysis between data collected in testing and calculated by the model showed an ideal effect in the calculation method, with a relative error of less than 15% in estimating erosion on sandy soil slopes. Experimental data showed that DPCM could effectively improve the stability and resistance to water erosion on sandy soil slopes, and these results could be verified in the practical application of sandy soil slope reinforcement.