The invention discloses a steel strand mechanical testing device in an ultra-low-temperature environment and a testing method thereof. The device comprises a universal testing machine and a low-temperature box arranged between an upper jaw and a lower jaw of the universal testing machine; through holes which are opposite to each other and are used for the passing of a connecting component connected with a tested steel strand are respectively formed in the upper surface and the lower surface of the low-temperature box; the connecting component is connected with the two ends of the tested steel strand and consists of a force transfer rod, a connection self-locking component and a self-locking clamp head. According to the steel strand mechanical testing device in the ultra-low-temperature environment and the testing method thereof, the aim of completely arranging the tested steel strand in the low-temperature box is fulfilled by utilizing the connecting component for the tested steel strand; the temperature of the steel strand is distributed uniformly; mechanical parameters can be tested accurately; the test accuracy is improved, wherein the connecting component comprises the connection self-locking component, the self-locking clamp head and the force transfer rod, so that the overall device is simple in structure, easy to machine and convenient to detach and can be reused; moreover, the testing method is simple and is convenient to operate; the test accuracy is improved, and the test efficiency is guaranteed, and therefore, the device and the method can be widely applied to steel strand mechanical tests.