Reliable pull-up detection and prevention of misuse warning in the operation of heat-drop bus has been a difficult problem of daily operation and maintenance of substations.In this paper,a computation method is proposed based on research on influence of system operation and contact resistor on switch currents and unbalance of three phase.With state information in bus coupler and transmission line collected,a remote forewarning method for bus switching is studied based on switcher current and degree of three-phase unbalance.The technology combined with bus protection and background monitoring system,is integrated in the model of bus protection of traditional substation or smart substation,and installed in five-anti system to forewarning the danger in bus switching.The reliability of the method is verified by simulation and operation data of actual substation.