The present invention relates to a rebar mechanical splicing inside-sleeve length detection tool. The rebar mechanical splicing inside-sleeve length detection tool includes an induction head and a host; the induction head is connected with the host through a data line; the induction head is a clamping head; the clamping head comprises a function head and an auxiliary head; the joint of the function head and the auxiliary head is provided with a torsion spring; a circle of heating resistance wire is wound on the inner surfaces of the function head and the auxiliary head; a heating power supplyis arranged inside the auxiliary head; the heating power supply is connected with the heating resistance wire; an infrared sensor is arranged in the inner cavity of the function head; and the data line of the infrared sensor is connected with the host. The rebar mechanical splicing inside-sleeve length detection tool can be used for accurately detecting rebar mechanical splicing inside-sleeve length. According to the rebar mechanical splicing inside-sleeve length detection tool of the invention, whether an end head of a rebar inside a sleeve tightly contacts with the sleeve or is separated from the sleeve by a certain gap can be displayed by means of an imaging principle; whether the rebar mechanical splicing inside-sleeve length meets requirements can be detected; field personnel can be facilitated to perform detection at any time; and therefore, rebar mechanical splicing quality can be ensured. The host is provided with operation buttons and a display screen, can display data directly, has a printing function, and is provided with an interface connected with a computer.