  The type-II seesaw model can explain neutrino masses and address the baryon
asymmetry problem of the Universe simultaneously. In this letter, we explore
its phase transition and the resulting gravitational wave signals. We find a
strong first-order electroweak phase transition generically prefers a
relatively light triplet in the $300\sim500$GeV range, which is ideal for
collider searches and can generate gravitational waves within the sensitivity
reach of BBO and Ultimate-DECIGO. While above $\sim$1TeV where a future 100TeV
$pp$ collider will play a key role in model discovery, we integrate out the
triplet and perform a global fit analysis of this model at various future
colliders. A lower bound in the $10^{-3}\sim10^{-2}$eV range on the triplet
vacuum expectation value is obtained, which is comparable to or even better
than that from current $\mu\to e\gamma$ experiments depending on the lightest
neutrino mass.
