The impact of premixed n-butanol mixture on the heat release rate was investigated based on a modified light-duty diesel engine. The results show that reactivity stratification is formed in the cylinder through n-butanol port fuel injection (PFI) and diesel direct injection (DI). The initial heat release rate of the diesel/butanol dual-fuel combustion is restrained due to the low ignitability of butanol and the high volatility. Because of the auto-ignition of diesel, premixed n-butanol undergoes a high-temperature reaction, which has an active influence on the heat releasing of diesel/butanol dual-fuel combustion. With the increase of the amount of premixed n-butanol injected, the heat release rate in the initial combustion period has a critical value. When the n-butanol injection quantity is less than 13 mg/cycle, the initial heat release rate of dual-fuel combustion is lower than the pure diesel combustion because the lean premixed n-butanol/air mixture limits the flame propagation. When the fuel injection rate of n-butanol is higher than 13 mg/cycle, the heat release rate is accelerated, leading to obvious flame propagation.