A method, a device and an optical communication system for online calibration configurable site wavelength dependent attenuation are provided. The method comprises obtaining single wave emitted light power to be calibrated wavelength transmitted from an optical amplifier at a transmitting end (S101); performing the configurable site wavelength through configuration so that light wave to be calibrated wavelength can penetrate configurable site to reach an optical amplifier at a receiving end, and detecting light wave incident optical power reaching the optical amplifier at the receiving end (S102); and according to the single wave emitted light power to be calibrated wavelength and light wave incident optical power reaching the optical amplifier at the receiving end, obtaining wavelength dependent attenuation of the wavelength to be calibrated (S103). The method can obtain and update WDL in real time in a network starting stage and an expansion stage, so as to update the network state timely and ensure the high communication quality. In addition, the solution can realize the wavelength dependent attenuation online calibration completely based on the prior network environment, and does not need to add any hardware, which effectively controls the cost increase.