  We investigate the formation history of massive disk galaxies in
hydro-dynamical simulation--the IllustrisTNG, to study why massive disk
galaxies survive through cosmic time. 83 galaxies in the simulation are
selected with M$_{*,z=0}$ $>8\times10^{10}$ M$_\odot$ and kinematic
bulge-to-total ratio less than $0.3$. We find that 8.4 percent of these massive
disk galaxies have quiet merger histories and preserve disk morphology since
formed. 54.2 percent have a significant increase in bulge components in
history, then become disks again till present time. The rest 37.3 percent
experience prominent mergers but survive to remain disky. While mergers and
even major mergers do not always turn disk galaxies into ellipticals, we study
the relations between various properties of mergers and the morphology of
merger remnants. We find a strong dependence of remnant morphology on the orbit
type of major mergers. Specifically, major mergers with a spiral-in falling
orbit mostly lead to disk-dominant remnants, and major mergers of head-on
galaxy-galaxy collision mostly form ellipticals. This dependence of remnant
morphology on orbit type is much stronger than the dependence on cold gas
fraction or orbital configuration of merger system as previously studied.
