In order to investigate the mechanical extension property of high-temperature superconducting(HTS) cable at room temperature and 77 K, we have respectively studied the mechanical extension properties of 10 m HTS cable and 10 m copper core at room temperature and 77 K. The stress-stain curves and Young modulus of the samples at corresponding temperature have been obtained by experiments. Meanwhile, we have investigated the contraction of 10 m HTS cable and 10 m copper core in a cooling process from room temperature to 77 K with tensile load of 2000 kg. The experimental results obtained in this paper could provide references to the determination of margin of cable-laying.