Abstract: Tight reservoirs are mostly developed by volumetric fracturing horizontal wells with the elastic drive of natural energy in China. As the production of horizontal well decreases rapidly, the primary oil recovery is less than 10%. To effectively supplement the formation energy is the key for stable production of the volumetric fracturing horizontal wells in tight reservoirs. Water huff-n-puff is an effective method for the supplementation of formation energy in tight reservoir for horizontal wells. In recent years, a lot of researches and practice have been performed on this method. This paper introduces the process of water huff-n-puff technology, outlines the research progress on the mechanism of water huff-n-puff from two aspects:micro-mechanism of imbibition oil recovery and influence law of pressure on imbibition oil recovery, and further systematically summarizes the influence law of reservoir properties and process parameters on the oil recovery of water huff-n-puff technology. To improve the oil recovery of water huff-n- puff, there are mainly three technologies, including the water huff-n-puff technology assisted by chemical treatment agent and enhanced by large displacement water injection, and inter-fracture asynchronous injection-production technology in the same horizontal wells. Through summarizing the research results from reservoir numerical simulation and process parameter optimization for water huff-n-puff, and analyzing field practise experience, this paper further proposes the future development trends of water huff-n-puff technology in tight reservoirs.