The invention relates to a pretreatment method for detecting boron impurities of slagging constituent, in particular to a pretreatment method for detecting boron impurities of titanium dioxide, which comprises the following steps: adding nitric acid and hydrofluoric acid into titanium dioxide so as to enable titanium dioxide to be dissolved, adding propylene glycol to be subjected to complexation with boron after diluting acidity through adding water, performing oscillation and standing, extracting boron complex by adopting normal hexane, then performing standing stratification, heating the normal hexane phase of the upper layer till full volatilization, adding nitric acid into the residue, then adding water for dilution, and finally performing detection after constant volume. The method has the advantages as follows: the gathering function of boron is obvious, few titanium ions exist in the solution to be detected, the influence of the matrix effect is greatly reduced, and the extraction yield is greater than 90%; the recovery of the boron can be improved to more than 95% from 80-82%, and the method detection limit of the boron is reduced to 0.5 ppm from 2 ppm; in addition, the pollution to the sampling system of an apparatus is avoided, and the method is simple and easy to operate, and improves the detection accuracy.