The invention discloses a self-expanding patching device and method for an oil well casing. The method comprises the following steps: taking a memory alloy pipe of which the outer diameter is smaller than the inner diameter of the casing as a patching pipe substrate; fixing a rubber seal ring on the outer wall of the memory alloy pipe via a temperature-resistant adhesive, so as to obtain a patching pipe; lowering the patching pipe to the damaged position of the casing with an underground tool releasing device; heating the patching pipe with temperature in a well, so that the memory alloy pipe expands fast when temperature of the memory alloy pipe reaches a training temperature, and is tightly patched to the inner wall of the casing, that is, the casing pipe is repaired, wherein the training temperature depends on the damaged position of the casing. As the patching pipe deforms in one direction, the sealing and the blocking are reliable, the strength is high, and the internal pressure resistance and the external collapse resistance of the damaged casing can be improved obviously after the casing is repaired. Moreover, as the patching pipe has a self-expanding property at a set temperature, the casing can be repaired without taking measures such as hydraulic expansion or gunpowder explosion. Therefore, the operation is convenient, the construction is safe, the flaking off possibility is low, and the service life is long.