Contact and non-contact sag sensing technologies are studied in this paper. Performances of three sag sensors in low temperature are compared and the results show that laser ranging sag sensor has small measuring error especially in cold condition. Based on the results, sag online monitoring system of transmission line under severe cold condition is designed, which has applied in Hulun Buir, Hainal, Manzhouli. From the monitoring results, designed system in this paper has higher no fault rate than normal system, as well as no interrupt and packet loss, which indicates high stability and reliability. Results in this paper can offer clues to the design of online monitoring system under severe cold condition to prevent and reduce transmission line accident, which has great social and economic effects.