Thermal fluctuation caused by incomplete mixing of the coolant in the connecting pipes of liquid lead–bismuth reactor can generate thermal stresses on the solid components of pipes surface, posing a threat to structural safety. The characteristics of turbulent heat transfer in a lead–bismuth pool differ significantly from those in water, and the thermal fluctuation behavior in water may not be applicable to liquid lead–bismuth. Therefore, it is necessary to conduct research on the mechanism of thermal fluctuation in liquid lead–bismuth. Focusing on a T-junction structure, this study investigated the distribution of temperature, flow velocity, and turbulence intensity of liquid lead–bismuth using Computational Fluid Dynamics (CFD). Fourier transform was applied to obtain the Power Spectral Density (PSD) at various monitoring points. The results indicate that the thermal fluctuation during non-isothermal mixing of liquid lead–bismuth in the T-junction is dominated by vortices in the shear layer. The thermal fluctuation on the inner wall of the main pipe downstream of the mixing zone is caused by the thermal fluctuation of near-wall fluids. As the velocity ratio of hot to cold fluids increases, the mixing pattern in the T-junction transitions successively from wall jet, deflected jet to impinging jet, resulting in a reduction in the degree of thermal fluctuation.