The invention discloses a measurement method of dielectric loss of a cable, and belongs to the power transformation field and the measurement and testing field. The method solves the problems that inthe prior art, insulation status of cables cannot be diagnosed, insulation aging levels of the cables cannot be evaluated, and low efficiency of detection on the cables is caused. A relationship between an attenuation coefficient beta and the dielectric loss tan[delta] is invented and constructed, the attenuation coefficient delta is used to characterize the dielectric loss tan[delta], the dielectric loss tan[delta] is derived from an attenuation characteristic of a damping sine voltage wave, impacts of deviation voltage are eliminated, and inaccuracy due to voltage deviation and impacts on the dielectric loss tan[delta] are avoided. The method is mainly used for diagnosis on cable insulation performance.