Abstract: This paper reviews the in-orbit background of the Hard X-ray Modulation Telescope (HXMT) over the first six years, including the geographical distributions, spectral and temporal characteristics, as well as the long-term evolution of the in-orbit background of each payload. In addition, we also review the estimation methods for the in-orbit background of each payload of Insight-HXMT, providing a comprehensive introduction to the strategies for background estimation and the accuracy of the estimation. Overall, the in-orbit background of Insight-HXMT is consistent with expectations, and the background models for each payload can reliably estimate the spectrum and light curve of the in-orbit background.