Based on SCADA information， the soft realization idea for dynamic thermal rating（DTR） is presented， which establishes continuous estimation model of transmission line temperature and parameters of its heat balance differential equation（HBE）. Firstly， based on the explicit analytic physical rules for relationship between conductor's resistance and temperature， an extended estimation model of line resistance is established with measurements of the line， then the transmission line's temperature can be obtained indirectly; Secondly， considering resistance change Markov property， the adjacent time priori information of resistance is used to modify the estimation result so as to improve its accuracy; Finally， with the known transmission line temperature series， a fading-memory recursive least square method is used to estimate parameters of reduced HBE， hence， the function of DTR hardware is realized indirectly through electric measurement of transmission line. The feasibility of the proposed method is verified by the tested data from Yantai grid of Shandong province.