Abstract: A measuring-wavelength apparatus includes a beam splitter (2), a first optical sensor (6), a second optical sensor (7), a signal-attenuation component (5) and a processing unit (8). A measuring-wavelength method comprising: splitting an incoming light into a first beam and a second beam; transforming the first beam into a first output signal; attenuating the second beam by using a signal-attenuation component; transforming the attenuated second beam into a second output signal; calculating a difference between the first and the second output signals to achieve an optical loss of the light; and in view of the optical loss, looking up a reference table to obtain a wavelength of the light.