The invention relates to a signal light power detection instrument and a detection method, the instrument is used to detect the power of a light signal transmitted in an optical network, wherein, a noise source exists in the optical network and the instrument is positioned at the receiving terminal in the optical network and includes: a photoelectric conversion and amplifying device, which is used to receive the light signal, convert the signal into an electrical signal and to amplify the electrical signal; a filter circuit, which is used to receiving the amplified electrical signal, to conduct high-pass filtering to filter the low frequency noise in the amplified electrical signal, and to output a high-frequency signal; and a high-frequency power detection circuit, which is electrically connected with the filter circuit, used to detect the power of the high-frequency signal to determine whether a high-frequency signal is available and/or the power value of the high-frequency signal. By making use of the filter circuit and conducting detection at the receiving terminal, the invention doesn't need establish a protocol among nodes, and doesn't need special light modulation technology. The invention has the advantages of strong adaptability, small instrument volume, low cost and fast scanning velocity, and can realize the same signal detection effect depending on the actual stateof different systems.