Abstract: Giant clam is one of the most important bivalves, and performs a significant ecological role in maintaining the balance of coral reef ecosystem. In China, giant clams are distributed in the Penghu Islands and south of Hainan in the South China Sea. Since the 1990s, the giant clam population surrounding Hainan Island has been greatly reduced, of which fishing and marine tourism are considered as the two main factors. In order to evaluate the influence of illegal fishing and marine tourism on giant clam population, this paper studied the distribution, density and population structure, as well as their influence factors, of Tridacna noae at Wuzhizhou Island (WI) (mainly affected by marine tourism) and Sanya Coral Reef National Nature Reserve including Yalong Bay (YB), Luhuitou Peninsula (LP) and Dongmao-Ximao Island (DXI), which are mainly affected by illegal fishing in coastal waters of Sanya. Our study showed that (1) the densities of T. noae in YB and WI were 2.00×10^–2 and 1.92×10^–2 ind./m², respectively, which was significantly higher than that in LP and DXI (P<0.05); Moreover, the depth of T. noae distribution in WI was mainly 2—4 m below mean sea level, while the depth of T. noae distribution in YB was 6—8 m below mean sea level. (2) Four sizes of T. noae (shell length of <5 cm, 5—10 cm, 10—15 cm, and >15 cm) were found in WI, indicating that its population structure in this sea area was reasonable. (3) The favorite substrate of T. noae was the living coral reef, followed by the rock, then the sand. (4) There was a significant correlation in RGB values between T. noae mantle and background colors (P<0.05), and a stronger correlation was observed in Sanya National Coral Reefs Reserve group. In summary, illegal fishing significantly affected the distribution and mantle color of T. noae, compared to marine tourism. This result will help the giant clam conservation.