The invention discloses a method and system for obtaining the fault severity of a multi-circuit transmission system in the same tower, and belongs to the technical field of power systems. The method comprises the steps of determining parameter per-unit values of various lines of the multi-circuit transmission system in the same tower of the same sending and receiving ends of the same voltage level; determining the impedance of the various lines according to the per-unit values, and selecting a line 1 and line 2 with the smallest impedance value as fault occur lines; obtaining severity indexesof different types of faults on the line 1 and line 2; comparing the fault severity indexes, and sorting by value sizes, wherein the fault corresponding to the maximum index value is the most seriousfault. The method utilizes the parameters of the lines, can directly calculate the severity indexes of different types of the faults of the multi-circuit lines in the same tower of the same sending and receiving ends of the same voltage level, directly sorts the fault severity indexes, can directly judge the most serious fault type, and is of great significance to guide the safe and stable checking of the power systems.