The intervisibility testing algorithm in the virtual environment that based on the LOS (line of sight) is researched on. After analyzing, the SO (sieve overlap) algorithm, which is more universal and efficient than other point to point intervisibility testing algorithm, is improved to fit more for the requirements of large-scale virtual environment and simulation. SO algorithm uses Bucket List and Singly Threaded Ternary Tree to manage and search the terrain polygon. The searching scale of Bucket List is reduced based on the AOI(area of interestiong).This makes the point to point intervisibility testing algorithm more real and efficient and gives the environmental infomation to the dynamic entity in the large scale virtual environment more efficiently.The space and time efficiency of the algorithm SO^*and the SO is also compared.