Method for detecting degradation of a turbine engine, by means of a computer, by monitoring the performance of said turbine engine, the turbine engine comprising a plurality of functional modules (M1-M5), each functional module (M1-M5) being characterised by at least one health parameter (ΔSE1- ΔSE5, ΔSW1- ΔSW5) representing degradation of the functional module (M1-M5) of the turbine engine, the global aging of the turbine engine being characterised by an aging parameter (λ), in which method: - a plurality of physical parameters of the turbine engine are measured in order to form a current performance index (Y COUR) of the turbine engine; - a plurality of degraded performance indices (Y DEG1 - Y DEG5) of the turbine engine are calculated by means of a thermodynamic model of the turbine engine, assuming for each degraded performance index (Y DEG) that only one functional module of the turbine engine has degraded, each degraded performance index (Y DEG1 - Y DEG5) being associated with a functional module (M1-M5); - each degraded performance index (Y DEG), corresponding to a single degraded functional module among all the functional modules (M1-M5) of the turbine engine, is obtained by simulating a performance index of the turbine engine for which the health parameter (ΔSE, ΔSW) of the degraded functional module and the aging parameter (λ) are variables, the health parameters of the other, non-degraded functional modules being considered to be healthy health parameters; - a plurality of cost indices are calculated, each cost index corresponding to the difference between the current performance index (Y COUR) and each degraded performance index (Y DEG1 - Y DEG5), varying the variables (ΔSE, ΔSW, λ) of the degraded performance index (Y DEG1 - Y DEG5) so that the difference is minimal, each degraded performance index (Y DEG1 -YD EG5) being associated with its cost index (J1-J5); - the optimum cost index (J opt) corresponding to the one with the lowest value among all the cost indices (J1-J5) is determined; - degradation of the module of the turbine engine, the degraded performance index of which is associated with the optimum cost index (J opt), is detected. - a plurality of physical parameters of the turbine engine are measured in order to form a current performance index (Y COUR) of the turbine engine; - a plurality of degraded performance indices (Y DEG1 - Y DEG5) of the turbine engine are calculated by means of a thermodynamic model of the turbine engine, assuming for each degraded performance index (Y DEG) that only one functional module of the turbine engine has degraded, each degraded performance index (Y DEG1 - Y DEG5) being associated with a functional module (M1-M5); - each degraded performance index (Y DEG), corresponding to a single degraded functional module among all the functional modules (M1-M5) of the turbine engine, is obtained by simulating a performance index of the turbine engine for which the health parameter (ΔSE, ΔSW) of the degraded functional module and the aging parameter (λ) are variables, the health parameters of the other, non-degraded functional modules being considered to be healthy health parameters; - a plurality of cost indices are calculated, each cost index corresponding to the difference between the current performance index (Y COUR) and each degraded performance index (Y DEG1 - Y DEG5), varying the variables (ΔSE, ΔSW, λ) of the degraded performance index (Y DEG1 - Y DEG5) so that the difference is minimal, each degraded performance index (Y DEG1 -YD EG5) being associated with its cost index (J1-J5); - the optimum cost index (J opt) corresponding to the one with the lowest value among all the cost indices (J1-J5) is determined; - degradation of the module of the turbine engine, the degraded performance index of which is associated with the optimum cost index (J opt), is detected.