Method for monitoring a degradation of a device on board an aircraft during the operation thereof, implemented by a computer, the degree of the degradation of the onboard device being defined by an abnormality score formed by the counting of occurrences of anomalies observed by a control system of the device during the operation thereof, the monitoring method comprising a step of comparing an abnormality score obtained for an observation sequence of a given length (n, t) with a decision threshold (ks) and a step of emitting an alarm in case the decision threshold (ks) is reached or exceeded, the decision threshold (ks) being automatically determined for a given alarm probability Pa, corresponding to the probability that an alarm is emitted in the course of the monitoring method although the onboard device is healthy, by means of the following steps: - a step of obtaining an abnormality score (r) on at least one reference sequence corresponding to flights of the aircraft without degradation and of length (m, tc) equal to a plurality of lengths (n, t) of observation sequences; - a step of adjusting a discrete probability law making it possible to retrieve the abnormality score (r) obtained on the reference sequence; - a step of calculating the decision threshold (ks) such that by applying the discrete probability law adjusted during the previous step to an observation sequence having the given length (n), the probability that an anomaly score greater than or equal to the decision threshold (ks) will occur a number of S times for N successive sequences is less than an elementary probability Peac of exceeding the threshold evaluated from the given probability Pa, N and S being able to be equal to one, the elementary probability Peac of exceeding the threshold evaluated on the basis of the alarm probability Pa being given by the formula: Peac = B S, N − S + 1 − 1 Pa wherein N is the number of observation sequences, S the number of times the threshold will have been exceeded of N successive sequences and B S, N − S + 1 − 1 is the inverse Euler beta distribution function of parameters S and N-S+1. - a step of obtaining an abnormality score (r) on at least one reference sequence corresponding to flights of the aircraft without degradation and of length (m, tc) equal to a plurality of lengths (n, t) of observation sequences; - a step of adjusting a discrete probability law making it possible to retrieve the abnormality score (r) obtained on the reference sequence; - a step of calculating the decision threshold (ks) such that by applying the discrete probability law adjusted during the previous step to an observation sequence having the given length (n), the probability that an anomaly score greater than or equal to the decision threshold (ks) will occur a number of S times for N successive sequences is less than an elementary probability Peac of exceeding the threshold evaluated from the given probability Pa, N and S being able to be equal to one, the elementary probability Peac of exceeding the threshold evaluated on the basis of the alarm probability Pa being given by the formula: Peac = B S, N − S + 1 − 1 Pa wherein N is the number of observation sequences, S the number of times the threshold will have been exceeded of N successive sequences and B S, N − S + 1 − 1 is the inverse Euler beta distribution function of parameters S and N-S+1.