  Time-reversal-invariant topological superconductors have a full paring gap in
the bulk and gapless Majorana states at the edge or on the surface. Here, we
theoretically propose topological superconductivity in a doped quantum spin
Hall insulator. We study the pairing symmetry of doped Sn$X$ film within a
two-orbital model, and show that it realizes a topological superconductor when
the intraorbital attractive interaction is stronger than the interorbital
interaction, which is generic for a doped quantum spin Hall insulator with
strong spin-orbit coupling. Edge channels are studied in a tight-binding model
numerically. Finally, we discuss the possible topological superconductivity in
Sn$X$ film by comparing to $3d$ superconductivity in bulk Sn.
