In ethanol/diesel dual-fuel engines, premixed ethanol/air atmosphere could affect the ignition delay of diesel spray as well as the following soot formation. In this paper, a reduced mechanism describing the chemistry of ethanol and n-heptane was composed and validated with various experimental data. Based on this mechanism, simulations were conducted to investigate the combustion characteristics of n-heptane spray under premixed ethanol/air atmosphere in a combustion vessel. The effects of equivalence ratio of premixed ethanol/air atmosphere, ambient temperature and charge cooling effect of ethanol vaporization were investigated. Under premixed ethanol/air atmosphere, the results showed that high temperature ignition first occurred in the regions with mixture near stoichiometric ratio, rather than regions with largest n-heptane content (i.e. highest fuel reactivity). This is mainly because the vaporization of direct injected n-heptane decreased the gas temperature and consequently restricted the low temperature reaction. While the equivalence ratio of premixed ethanol/air atmosphere was further increased, the mixture near stoichiometric ratio exhibited lower fuel reactivity, which consequently resulted in prolonged ignition delay. It was also found that premixed ethanol could inhibit the soot formation of n-heptane spray under low ambient temperatures while contributed to significantly increased soot emission under high ambient temperatures. Moreover, the simulated results showed that the cooling effect of ethanol vaporization also played an important role in soot reduction of dual-fuel combustion.