The invention discloses a prestressed anchor device and technique with blast bore expanding and automatic filling functions and for a weak stratum. The prestressed anchor device comprises an anchor rod body, wherein an igniter is mounted at the tail end of the anchor rod body, and a tray and a nut are mounted on the outer circle of the outer end of the anchor rod body. An anchor rod is anchored and mounted through a combined type anchor cartridge. The combined type anchor cartridge comprises an explosive cartridge III, an anchor cartridge II and an anchor cartridge I, and three corresponding filling sections can be sequentially formed outwards at the bottom of a drilled hole after the anchor rod is mounted on the field and before blasting is conducted. The igniter is cylindrical, a detonation material is mounted on the inner circle of the igniter, and barb structures are arranged on the outer surface of the igniter. The igniter can ignite explosives on the filling section of the explosive cartridge III, and a local blasting cavity is formed around the wall of the drilled hole. Meanwhile, the igniter can be opened automatically, so that the barb structure are engaged with a rock-soil body around; and under the effect of the high-temperature energy generated when the explosives explode, the filling section of the anchor cartridge II can expand, then the blasting cavity is filledwith the expanded anchor cartridge II, and thus an internal anchor structure is automatically expanded.