Wavelength division multiplexing (WDM) technology has been shown to effectively increase the capacity of optical communications with minimal channel crosstalk in the near-infrared light band. However, to date, WDM systems in the blue-green band support a limited number of wavelength channels with coarse channel spacing. This is because no multi/demultiplexers (MUX/DeMUX) exist that use fine wavelength spacing for visible light. To clarify this issue, this study reviews the development of WDM in the visible band and discusses the recent development of dense blue-green WDM based on integrated optical phased array DeMUX. Finally, the study summarizes and prospects the development trends of blue-green WDM.