A ball or half-ball valve with drain-proof mechanism comprises a body; inner side of the body being formed as a chamber and a through hole communicated to the chamber; an annular seat received in an annular trench of a wall of the through hole; a drain-proof unit; a valve unit installed to the inner side of the annular seat; the valve unit having an axial hole for receiving the stem; and a cover having a size corresponding to that of the through hole of the body; the cover being locked to an outer side of the through hole. The drain-proof unit has three different structures. The elements of the structures are replaceable with each other for being installed to the valve. Each mode is suitable for different environments. In the present invention, the cover can be detached easily for repairing or maintaining the elements therein. It is easy and convenient.