In point of view of the system, the characteristic of leak current fault diagnosis in launch vehicle controlling system is analyzed. It is presented that the fault is a connecting way for sytem factors. By building some basic concepts such as componts, syptoms, observation and faults, the system model is turned into graph, where nodes represent components and edges represent fault transmission among components. Besides,designing rule, that is, bisection rule for diagnosing alglorithm is given based on the information theory. Operating feasibility and entropy function characteristic are analyzed using component trade-off value to substitute component entropy, giving fault problem-solving way based on the drawing model.