Abstract: The Hainan Island is one of the provinces where tropical cyclones made landfall frequently. There are 2.7 landfall tropical cyclones in average annually only less than Guandong province. A severe tropical strom, Fitow (0114) crossing Qiongz hou Strait would has a great structure change due to the topographic forcing. A mesoscale convective vortex system (MCS) could be appeared in the southwest periphery of a storm Fitow (0114) where is the convergence area between the mountain range and the NW flow of Fitow. This small vortex (MCS) would affect the struct ure change and motion of Fitow. Intensive conventional observations, satellite i mages and radar reflectivity data have identified the genesis and existence of t he MCS in the northwest side of the mountain range. The numerical simulation was performed with PSU/NCAR MM5V3 model to study the genesis of the small vortex of MCS. The results of the simulation and filtering analysis show that a mesoscale convective vortex system (MCS) appeared clearly in a topographic convergent area caused by the peripheral flow field of Fitow and the mountain range which was favorable to form the MCS when Fitow was passing through the Qiongzhou Strait. The results also show that the MCS only occurred in the layer below the mountain altitude and disappeared in the level higher than the mountain. Higher altitude of the mountain is more favorable to the formation of MCS than the lower one. Many studies showed that tropical cyclone motion can be influenced by some mesos cale vorticies or topographic induced low in its periphery when it made landfall . One of the results from the simulation indicates that the MCS southwest to the Fitow's center caused the storm to turn direction towards southwest. It seems to be some attraction existed between the MCS and the Fitow. The another results showed that the Fitow would move towards northwest if without the MCS or the altitude of the mountain range in Hainan island is zero (no influence of the topogr aphy).