Abstract: The hydrocracking of C_6~8 mixed chain hydrocarbons(mixed hydrocarbons) and o-xylene over HZSM-5 zeolites with different acidity was studied in a fixed-bed down-flow reactor, and the effects of reaction parameters such as temperature, WHSV and H₂/CH volume ratio were investigated after the catalyst reached stable performance. Hydrocracking of mixed hydrocarbons was accompanied by aromatization over HZSM-5 zeolite. The hydrocracking was the main reaction with catalyst of weak acid sites, and the hydrocracking products were main normal alkanes, while both methane and isoalkane were pretty low. However, with HZSM-5 zeolite of strong acid sites, the main reaction was aromatization at the beginning. After a certain time, a stable catalytic performance was reached, presenting similar product selectivity to that of weak acidic HZSM-5. The hydrocracking of mixed hydrocarbons displayed obvious temperature effect, while both the WHSV and H₂/CH volume ratio had a small effect. At 380℃,3.0MPa,WHSV of 1.02h^-1 and H₂/CH volume ratio of 1000, the conversion of mixed hydrocarbons kept above 99% within 100h, and the stable hydrocracking selectivity was above 95%. o-Xylene hydroconversion was found to be a linear combination of three reactions: hydrocracking, isomerization and disproportionation. Over the strong acidic HZSM-5, the yield of hydrocracking products was higher than that of the weak acidic HZSM-5, and over both HZSM-5 the hydrocracking product distribution was similar to that of mixed hydrocarbon hydrocracking. At 380℃, 3.0MPa, WHSV of 1.3h^-1 and H₂/CH volume ratio of 1000, the stable hydrocracking selectivity was less than 10% for o-xylene hydroconversion.