The invention discloses a method for determining through-flow upper limit of high-voltage direct-current plastic insulation cable accessory. The method comprises following steps: building a finite element model; calculating the highest temperature of a wire core under different load currents and the maximal temperature difference allowed by the exterior surface inside each solid insulation layer; taking the highest long-term allowable temperature of the wire core and the maximum allowable temperature difference between inside and outside surfaces of each solid insulation layer as restricting conditions for the steady load flow of high voltage direct current cable attachment to get an initial current loading section; using the ideas of dichotomy to continuously narrow the current loading section and finally obtaining the steady load flow of high voltage direct current cable attachment. During the operation of high voltage direct current cable attachment, taking the calculated result of load carrying capacity as the through-flow upper limit of the attachment. In the invention, both the highest long-term allowable temperature of the wire core and the maximum allowable temperature difference between inside and outside surfaces of each solid insulation layer are taken as two restricting conditions, which meets the insulation characteristics of high voltage direct current cable attachment. With the method, it is possible to flexibly adjust the load carrying capacity of the attachment in accordance to exterior environment. The method makes full use of the transmission capacity of an electric cable line, therefore, increasing the utilization efficiency of an electric cable.