The invention provides a system and a method for improving the transmission performance of the cable line of a distribution network. The method includes the following steps: acquiring a first temperature value of each axial position of the sheath layer of a cable and a second temperature value of each axial environment position of the outer edge of the cable; estimating conductor temperature values of corresponding conductor sections according to the first temperature values and the second temperature values; acquiring the maximum conductor temperature value from the conductor temperature values; calculating the current maximum load safety redundancy of the cable according to the difference between the maximum conductor temperature value and a first preset temperature upper limit; increasing or limiting the load of the cable according to the current maximum load safety redundancy, and correspondingly forming one or more constraints to allow or limit the increase in the load of the cable according to whether the operation state (parameters such as partial discharge and temperature) of one or more types of cable joints connected with different sections of the cable and the bus-bar connecting components at the tail end is normal or not; and improving the transmission capacity to the maximum under the condition of ensuring the safety of line facilities.