A practical normalized stress intensity factor is stated in this paper on the basis of the evolution of cracks of cable wires, which fits the crack growth behavior for a semicircular crack and for straight crack. Example testifies that the fracture strength of a cable wire with a surface crack can be assessed conveniently and the result can be acquired with satisfactory precision by this convenient and practical method. And the fracture toughness criterion is more realistic for the fracture strength of the wire.