With the widespread application of the fifth-generation mobile communication system, mobile data traffic has seen rapid growth, bringing new challenges to traditional cellular networks. Ultra-dense networks (UDN) are one of the key technologies used in 5G communication to address user capacity and high-speed transmission. How to effectively solve the problems of inter-site interference, spectrum resource scarcity, and high energy consumption in UDN has become a major research topic, and resource allocation technology is essential in this regard. This article summarizes the spectrum resource allocation methods and applications in UDN through literature review. Firstly, the applicable scenarios, models, and various indicators of resource allocation in UDN are summarized and analyzed. Secondly, this paper systematically sorts and classifies recent research on resource allocation methods for UDNs, analyzing their applicability, advantages, and disadvantages. Additionally, the paper discusses the future development of spectrum resource allocation methods for UDNs.