An ultra-dense wavelength division multiplexing passive optical network can substantially increase the number of users and transmission capacity because it exhibits a channel spacing of only a few GHz， power budget of up to tens of dB， and other superior performance characteristics. Herein， the recent progress on the ultra-dense wavelength division multiplexing passive optical networks at home and abroad was comprehensively reviewed based on several key research directions， such as the improved spectral efficiency， simplified coherent receivers， photonic integrated device application， and optimized digital signal processing. Moreover， possible research directions and development trends were discussed.