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 obtaining parameter per-unit values X of each line of a multi-circuit transmission system in the same tower of different voltage levels and initial power per-unit values P of each line; calculating the ratios P/X of the power and impedance per-unit values of each line, and selecting two-circuit lines of different voltage levels with the largest ratio as fault occur lines; calculating the severity indexes of the different types of faults of the multi-circuit transmission system in the same tower of the different voltage levels; comparing the fault severity indexes, and sorting by index value sizes, wherein the fault corresponding to the maximum index value is the most serious fault. The method utilizes the parameters of the lines, can directly calculate the severity indexes of different types of the faults, directly sorts the fault severity indexes, directly judges the most serious fault type, and is of great significance to guide the safe and stable checking of the power systems.