Black carbon aerosols affect the shortwave and longwave radiation in climate in a strong yet uncertain way. In aging process, black carbon particles coated by co-emitted aerosols tend to reduce the shortwave radiative forcing of freshly emitted black carbon at the top of atmosphere (TOA), however, this effect is still unclear in the longwave range. Here in this work, we investigate the effect of black carbon aging on longwave radiative forcing. The freshly emitted black carbon aerosols are simulated to be fractal aggregates consisting of hundreds of small spherical primary particles, and these aggregated black carbon aerosols tend to be fully coated by the large sulfate particles after aging. The optical properties of these freshly emitted and internally mixed black carbon aerosols are simulated using the numerically exact superposition T-matrix method, and their longwave radiative forcings are calculated by the radiative transfer equation solver. The results indicate that the black carbon longwave radiative forcing at TOA is remarkably amplified (up to 3) by coating the large sulfate particles, while the black carbon shortwave radiative forcings decrease during their aging. Moreover, the thicker sulfate coatings tend to increase the longwave radiative forcings of black carbon aerosols at TOA. These findings should improve our understanding of the effect of black carbon aging on their longwave radiative forcings and provide guidance for assessing the climate change.