The temperature fluctuations induced by incomplete mixing of coolants with different temperature may cause thermal fatigue at the components of the lead-based reactor core outlet. Thus the accurate analysis of the phenomenon is very crucial for reactor safety operation. In this paper, the temperature fluctuations of the lead-based reactor core outlet were simulated by using large eddy simulation (LES) method in the simplified core outlet models. In order to analyze the temperature fluctuation sensitivity for the fuel assembly design parameters, such as the fuel assembly size and the gap between two adjacent fuel assemblies, five geometry models were constructed with different fuel assembly design parameters. The time histories of temperature fluctuations at different monitoring points on the center of three fuel assemblies were obtained. Then the amplitudes and the power spectrum density (PSD) of temperature fluctuations were analyzed, in order to compare temperature fluctuations of different geometry models at the same locations of core outlet. Finally the distribution characteristics of core outlet temperature fluctuations were obtained in axial directions, and the temperature fluctuation sensitivity with fuel assembly parameters was also analyzed based on the amplitudes, PSD and the normalized root-mean square temperature analysis. It is found that the temperature fluctuation intensity is enhanced with the increase of the gap size between adjacent fuel assemblies and the opposite edge width of each fuel assembly. The analysis results could provide important references for optimized design and engineering guidance of lead-based reactor. (C) 2018 Elsevier Ltd. All rights reserved.