The invention discloses a test device and test method for determining the dynamic response of a composite crossarm under a line breaking working condition, so that the dynamic load can be convenientlyapplied, the strain of the crossarm dangerous point can be accurately tested, and the data can be correctly processed in a crossarm test. According to the invention, a numerical simulation method isused for determining the section and position for distributing a strain gauge, the strain gauge is arranged at a dangerous point, and the response of the dangerous section is recorded by using a dynamic strain meter. According to the invention, a connecting thread rod is used for installing weights, which reduces the oscillation during the loading process, and also provides convenience for the actual loading; the dynamic strain meter is used for recording data in real time, therefore, the accuracy and reliability of the data are ensured, and the crossarm self-vibration frequency can be obtained from a strain fluctuation response diagram; a pull rope quick detacher is used for releasing the line breaking load, therefore, the operation is simple and safe, and the instantaneous release for detaching can play the line breaking effect; and the invention is simple to install and adjust in practical use, and is convenient for the use of operators in various projects.