All coral atolls consist of several facies zones. Tidal inlet, which is the channel to connect the lagoon and the sea, has different geometric shapes. According to different angle and principle, coral atolls have been classified into various types. In order to identify the open type of atolls quantificationally, two geometric parameters of tidal inlet are regarded as new metrical factors to construct a new expression. The new description function is the ratio of tidal inlet’s width to atoll’s midline. Finally, the threshold division of the atolls’ type is formed on the basis of the new expression. The threshold result shows that the open degree of the enclosed atolls is 0, the contiguous enclosed atolls is between 0 and 0.06, the half-opening atolls’ is between 0.06 and 0.25, and the opening atolls’ is between 0.25 and 1. Thus, the new function can be used to reflect the openness of different atolls for development regularity of coral atolls in the South China Sea.