A polarization-dependent loss determining method, wherein the method is applied to a detection system comprising an optical device, wherein the detection system is configured to directly detect optical-path output powers of two polarization principal axes of the optical device, and the method comprises: obtaining two groups of optical powers within first duration, wherein an optical power satisfying a same power constraint exists in each of the two groups of optical powers within the first duration, the two groups of optical powers are optical-path output powers of the two polarization principal axes of the optical device; selecting at least one group of target optical powers, wherein the target optical powers are optical powers which are selected to satisfy the same power constraint from the two groups of optical powers, wherein each group of target optical powers comprises a first target power and a second target power from the two groups of optical powers, wherein the power constraint is any one of a maximum output power, a minimum output power, an average value of output powers, and a weighted average value of output powers within the first duration, or a power which occurs with a predetermined probability; and determining a polarization-dependent loss PDL of the optical device based on the at least one group of target optical powers, wherein the obtaining two groups of optical powers within first duration comprises: splitting the optical signal input to the optical device into a first polarization signal and a second polarization signal, wherein polarization directions of the first polarization signal and the second polarization signal are perpendicular; converting the first polarization signal to obtain a third polarization signal, wherein polarization directions of the third polarization signal and the second polarization signal are the same; and detecting, within the first duration, a first output power of a signal with a specified energy proportion in the third polarization signal, and detecting a second output power of a signal with the specified energy proportion in the second polarization signal, wherein the two groups of optical powers are the first output power and the second output power. obtaining two groups of optical powers within first duration, wherein an optical power satisfying a same power constraint exists in each of the two groups of optical powers within the first duration, the two groups of optical powers are optical-path output powers of the two polarization principal axes of the optical device; selecting at least one group of target optical powers, wherein the target optical powers are optical powers which are selected to satisfy the same power constraint from the two groups of optical powers, wherein each group of target optical powers comprises a first target power and a second target power from the two groups of optical powers, wherein the power constraint is any one of a maximum output power, a minimum output power, an average value of output powers, and a weighted average value of output powers within the first duration, or a power which occurs with a predetermined probability; and determining a polarization-dependent loss PDL of the optical device based on the at least one group of target optical powers, wherein the obtaining two groups of optical powers within first duration comprises: splitting the optical signal input to the optical device into a first polarization signal and a second polarization signal, wherein polarization directions of the first polarization signal and the second polarization signal are perpendicular; converting the first polarization signal to obtain a third polarization signal, wherein polarization directions of the third polarization signal and the second polarization signal are the same; and detecting, within the first duration, a first output power of a signal with a specified energy proportion in the third polarization signal, and detecting a second output power of a signal with the specified energy proportion in the second polarization signal, wherein the two groups of optical powers are the first output power and the second output power. splitting the optical signal input to the optical device into a first polarization signal and a second polarization signal, wherein polarization directions of the first polarization signal and the second polarization signal are perpendicular; converting the first polarization signal to obtain a third polarization signal, wherein polarization directions of the third polarization signal and the second polarization signal are the same; and detecting, within the first duration, a first output power of a signal with a specified energy proportion in the third polarization signal, and detecting a second output power of a signal with the specified energy proportion in the second polarization signal, wherein the two groups of optical powers are the first output power and the second output power.