The invention discloses an in-service prestressed reinforcement local position effective stress detection device and a measurement method. The device fixes clamps on a sleeve via supports; the clamps are clamped on a to-be-measured prestressed reinforcement; a brake and a quality ball sleeve a guide rail; the quality ball is fixed by using the brake; one end of the guide rail is fixed in the sleeve, and the other end of the guide rail is tightly contacted with the to-be-measured prestressed reinforcement; a point light source is placed at the center of the inner wall of the sleeve; a photoelectric tape with a certain length is placed along the axial direction of the sleeve; and the point light source and the photoelectric tape are placed oppositely and connected with a data collector. The fact that the reinforcement has a different lateral stiffness under a different prestress level and thus difference exists in energy transfer characteristics during a collision and impact process with the outside is used, the mechanical energy change amount of the outside quality ball is used for capturing the difference, and on the premise of not breaking force transmission integrity of the reinforcement, the effective stress at the specified position of the in-service prestressed reinforcement can be detected.