For a low grade dolomite type fluorite ore in the Hebei province, it was observed that the depressant CK102, a mixture of sulfuric acid, sodium silicate and aluminum sulfate, can effectively inhibit the gangue mineral dolomite in the flotation of fluorite. However, the inhibition mechanism of the depressant is still unclear. In this paper, the flotation separation performance and underlying mechanism of CK102 inhibiting dolomite were investigated through mineral flotation tests, adsorption measurements, infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS). The flotation results showed that the inhibition effect of CK102 on dolomite flotation was much more remarkable than that of fluorite flotation under optimum conditions. Adsorption measurements revealed that there was competitive adsorption between the depressant and collector and that the adsorption of the depressant CK102 prevented the collector modified sodium oleate from adsorbing onto the surface of minerals. The FT-IR and XPS results showed that the co-oxygen cross-linked component of the depressant CK102 chemisorbed on the surface of dolomite; the CaSiO3 precipitation was generated from the reaction of CK102 with Ca2+ groups on the surface of the dolomite; Al2MgO8Si2 precipitation was also generated from Mg2+ reacting with the sodium silicate and aluminum sulfate of CK102. The above adsorptions and reactions enhanced the hydrophilicity of the dolomite surface and the dolomite was effectively depressed.