The mixed-acid systems of four Lewis acids (FeCl3, CrCl3, ZnCl2, and CuCl2) combining three Bronsted acids (H2SO4, HCl, and H3PO4) were evaluated for the decomposition of glucose to produce levtilinic acid (LA). The CrCl3-H3PO4 system had a strong synergic catalytic activity for the decomposition of glucose to LA. The effects of the ratio of CrCl3 and H3PO4 on glucose, fructose, and 5-hydroxymethylfurfural (5-HMF) decompositions were investigated. The mixed-acid system showed the strongest synergic catalytic activity for glucose, fructose, and 5-HMF decompositions when the ratio of CrCl3 in the CrCl3-H3PO4 system was 0.4-0.5. To probe the synergic catalysis mechanism of the CrCl3-H3PO4 system, the synergic catalytic activities of CrCl3-phosphates (KH2PO4, K2HPO4, and K3PO4) systems on glucose decomposition were also evaluated. The possible synergic catalysis mechanisms were proposed. This study provides insights for the synergic catalysis mechanism of hexose conversion to yield LA.