The link traffic time estimation for urban traffic is an important part of the route guidance system, it is a necessary condition to realize the multiple functions of intelligent transportation, which has important effect on traffic planning, traffic management and traffic control. From the perspective of the research content, this paper analyzes the studies of link travel time estimation of urban road net in detail, which are divided into model construction and optimization. On the basis of the previous work, it pointed out the challenges and developing trend of the research about the work of link travel time estimation. This paper puts forward some constructive suggestions for the future research of link travel time estimation.