In light of the global scope of coral reefs degradation, macroalgae cover is becoming increasingly common in reef regions. The overgrowth of macroalgae in reef regions hampers the growth, reproduction and recovery of coral colonies, primarily because: (1) macroalgae compete with corals for space and light, lowering the growth rate of corals, causing death through suffocation and inhibiting the engraftment of coral larvaes; (2) macroalgae negatively affect corals growth via physical abrasion and the release of allelochemicals when in direct contact with corals; abrasion causes coral polyps to shrink and influences the ingestion behaviour and calcification of polyps, while the release of allelochemicals into the surrounding environment of corals may cause coral bleaching and the damage of coral tissue. (3) The reproduction of corals is a highly energy consuming process. Any outside pressure which may drain a coral's stored energy will influence the fertility of the coral. In reef regions, the overgrowth of macroalgae breaks its competitive balance with corals, since, in response to the invasion of macroalgae, corals re-allocate energy needed for growth and reproduction to tissue repair and protection, causing a decrease in the quantity and quality of coral eggs, which decreases the energy allocated to reproduction; (4) The inhibiting effects of macroalgae on corals is the key factor to cause the degeneration of coral communities. Macroalgae influence the settlement and survival rate of coral larvas, thereby preventing the development of coral communities; (5) seaweeds accumulate sediments, release pathogens, and disrupt the growth of coral symbiotic microbes, indirectly influencing coral growth. This paper includes a brief overview of the primary competitive mechanisms (physical, chemical, and microbial) of macroalgae and corals. Understanding these competitive mechanisms will be very helpful when studying the interaction process between macroalgae and corals. We know much about the physical competition mechanisms, but the chemical and microbial mechanisms require more in-depth research; indeed, they are the hotspot of current research in coral reef sciences. In addition, this paper categorizes macroalgae according to their functional groups, lists the effects of three types of benthic seaweeds on corals, and summarizes the present status of coral reef degradation and the effect of macroalgae in areas with degraded reefs. Finally, it includes a review of the progress made by the latest research on the effect of macroalgae on corals, and suggests that our study should strengthen its research on the distribution and abundance of macroalgae species in the coral reef regions of the South China Sea in order to better evaluate the extent of macroalgae influence on the coral reefs of this region. Physiological, molecular, and ecological methods should be utilized to explore the mechanisms via which macroalgae influence corals at the cellular and molecular levels, in order to provide reference data for coral reef ecosystem conservation.