Cable ampacity is one of the important parameters of cable engineering design and operation, and its value is obtained under the condition of constant load current. The load current of a power system usually varies periodically during actual operation. In order to study the transmission capacity of cable under periodic load, a numerical calculation program for transient temperature field of cable is compiled based on the finite difference method, and the temperature rise test of a direct buried cable is carried out with a large current. The results of the test are in agreement with the numerical results. This transient calculation program is used to calculate the temperature rise of the cable core under 24h periodic load and determine the periodic negative load flow,the existing analytical formulas for the analytical calculation of periodic negative load flow are studied, and an analytical formula for calculating the periodic negative load flow of a direct buried cable is given, which improves the calculation accuracy of the analytical calculation of cyclic load flow.