The invention relates to a method and a device for monitoring avionics systems connected to a shared medium. A preliminary phase is aimed at determining the conditions of activation of each alarm of each avionics system connected to the communication medium. These conditions are related to each avionics system and to the state of the communication medium. The monitoring phase comprises a step of determining the state of the elements of the communication medium and a step (455) of evaluating an indicator of the state of the communication medium on the basis of the state of the elements of the communication medium and on the basis of a predetermined formal modelling of the communication streams of the communication medium. An alarm is selected and activated (470) depending on the evaluated state of the communication medium and depending on a predetermined modelling of the consequences of the evaluated state of the communication medium on the operation of the avionics systems.