A preparation method of a three-layer self-healing flexible strain sensor includes steps of: preparing an encapsulating layer composite, so as to obtain a concentrated solution; preparing a strain sensitive layer composite, so as to obtain a thick liquid; dropping the thick liquid on a glass substrate, and statically curing at a room temperature; dropping the concentrated solution on a cured film obtained in the S3, and statically curing at the room temperature; striping a cured filmed obtained in the S4 from the glass substrate, and drawing out two wires as electrodes; and dropping the concentrated solution on the other surface of the cured film obtained in the S3 with a same amount of S4, and statically curing at the room temperature for obtaining the three-layer self-healing flexible strain sensor. The three-layer self-healing structure strain sensor can be prepared without using a repair agent, but can achieve rapid self-repair.