The observational data of high redshift galaxies become increasingly abundant, especially since the operation of the James Webb Space Telescope, which allows us to verify and optimize the galaxy formation model at high redshifts. In this work, we investigate the merging history of massive galaxies at 3 < z < 6 using a well-developed semi-analytic galaxy formation catalog. We find that the major merger rate increases with redshift up to 3 and then flattens. The fraction of wet mergers, during which the sum of the cold gas mass is higher than the sum of the stellar mass in two merging galaxies, also increases from ∼34% at z = 0 to 96% at z = 3. Interestingly, almost all major mergers are wet at z > 3. This can be attributed to the high fraction (>50%) of cold gas at z > 3. In addition, we study some special systems of massive merging galaxies at 3 < z < 6, including the massive gas-rich major merging systems and extreme dense proto-clusters, and investigate the supermassive black hole–dark matter halo mass relation and dual active galactic nuclei. We find that the galaxy formation model reproduces the incidence of those observed massive galaxies, but fails to reproduce the relation between the supermassive black hole mass and the dark matter halo mass at z ∼ 6. The latter requires more careful estimates of the supermassive black hole masses observationally. Otherwise, it could suggest modifications of the modeling of the supermassive black hole growth at high redshifts.