Four representative pretreatment methods, water washing (WW), acid washing (AW), dilute acid hydrolysis (DH), and microwave assisted organosolv pretreatment (MOP), were adopted to alter eucalyptus compositions. The linkages between eucalyptus compositions and the resulting pyrolysis behaviors, kinetics, and product yields were investigated. The results showed that all four pretreatment methods evidently lowered the alkali and alkaline earth metal (AAEM) contents of eucalyptus, while DH and MOP significantly removed hemicellulose or lignin fractions. WW, AW, and DH reduced the activation energy for hemicellulose pyrolysis from 67.88 to 40.41-64.08 kJ/mol, whereas all four pretreatment methods improved that for cellulose pyrolysis from 71.83 to 77.08-205.47 kJ/mol. All four pretreatment methods effectively suppressed the formation of pyrolytic char, H2O, CO, CH4, aldehydes, ketones, and carboxylic acids, while evidently improving the levoglucosan yield. The maximum levoglucosan yield of 49.9% was obtained from eucalyptus pretreated by MOP. The levoglucosan yield was inhibited by the catalytic effects of AAEMs and the interactions between biomass major components.