Towing line safety is an important component of the towing safety. In order to ensure the towing line safety of the marine towage, we put forward the method of determining the length and sag depth of the towing line according to the water depth and sea condition, and based on the analysis of the relationship between the towing resistance and the length & sag depth of the towing line, which makes the towing line more flexible and without touching the seabed. By relative measures, we can avoid the damage or breaking of the towing line and thus achieve the objective of safe towing.