The live line measurement system of transmission lines with mutual inductance based on GPS is presented and analyzed. It points out that the harmonics' analyzing apparatus can also be used to measure the parameters of lines with mutual inductance if the function of accepting GPS signals is added. The principle and theoretical background are demonstrated. The proposed live measurement method for several parallel lines with mutual inductance is implemented on the project of parameters' live measurement of several 500 kV lines， which are parallel with the rebuilt Wang-NanⅠandⅡ lines in Liaoning Province. To deal with the difficulties， two new implementary methods are proposed.By these methods， the measuring results are obtained. By comparing with some results from other measuring methods， the validity of the results is proved indirectly. The results indicate that the measuring methods presented can meet the requirement of real projects.