Embodiments of the present invention provide a method, a device and a system for activating an on-line patch. The method comprises: positioning an address of a patch function and an entry address of a to-be-patched function; writing, in a middle segment, a long-jump instruction for jumping to the patch function based on the address of the patch function and the entry address of the to-be-patched function, where the middle segment is a storage space, which is located before or after the entry position of the to-be-patched function and can at least store one long-jump instruction; and modifying an instruction at the entry position of the to-be-patched function to a short-jump instruction for jumping to the middle segment, so as to jump to the middle segment after the short jump instruction is executed, and then to jump to and execute the patch function through that instructions in the middle segment are executed.