The invention provides a longitudinal tensile insulating dielectric electrical property measuring device and a measuring method thereof. The longitudinal tensile insulating dielectric electrical property measuring device is characterized by comprising a high voltage power supply, a support frame, a fixing seat disposed on the support frame, a limiting plate matched with the fixing seat and disposed on the lower portion of the fixing seat, a rotating shaft respectively connected with the supporting frame and the limiting plate and located below the limiting plate, an upper electrode member anda lower electrode member sequentially connected with the rotating shaft, a driving assembly for pulling the lower electrode member and a hollow sealing cavity covering the upper electrode member and the lower electrode member. The device has the advantages of achieving the measurement of breakdown field strengths of an insulating dielectric original sample and the sample after stretched in the directions of an upper electrode and a lower electrode, breaking through the limitation that the tested insulating dielectric sample can only be transversely stretched, and achieving the simulated measurement of the insulating dielectric sample under tensile and pressure states.