A signal filtering method, device and system, which are used for solving the problem of a low signal-to-noise ratio of a DOCSIS service signal in a DCA device. A first analogue-to-digital converter is used for converting a first radio frequency signal output by an optical receiver into a first digital signal. A digital domain filter is used for filtering off noise of a frequency band not used in the first digital signal converted by the first analogue-to-digital converter. A DOCSIS data access unit is used for converting a first downlink optical received by a DCA device into a second digital signal, combining same with the first digital signal output by the digital domain filter after filtering off the noise of the frequency band not used, and converting an obtained third digital signal into a second radio frequency signal and outputting same. A digital domain filter is used to filter noise of a frequency band not used in an optical receiver service signal, and the filtered optical receiver service signal is combined with a DOCSIS service signal, thereby improving the signal-to-noise ratio of a DOCSIS service signal.