To analyze the electromagnetic unbalance factor of 220 kV transmission cable line in Zhuhai-Macao Power Grid, this paper applies a method which incorporates Cason theory and electromagnetic transient simulation (EMTP) simulation technique. Firstly, the comparison of cable impedances derived from Carson theory and EMTP simulation is conducted. Then, according to the EMTP simulation model from one practical transmission cable line, impact factors of cable electrical parameters are analyzed. At last, calculations from three different factors (distance between phases, cross-bonding mode of metal sheath and arrangement of phases) are made to obtain the main cable electromagnetic unbalance factor. Results show that there only exists small cable line impedance errors between the Carson model and EMTP analysis. Analyses demonstrate that the metal sheath cross-bonding and ground resistance of the cable would partly impact the cable sequence impedance. The metal sheath cross-bonding and the distance between phases show great influence on the negative-sequence unbalance factor. For the analyzed transmission cable line between Zhuhai and Macao, when using ABC/CBA cable arrangement, the electromagnetic unbalance could be the minimum. This work is supported by National Natural Science Foundation of China (No. 51707047), Science and Technology Program of Guangdong Power Grid Co. (No. GDKJQQ20161098), and Science and Technology Project of Shenzhen (No. JCYJ20170811154143968).