This paper was concerned with the unsteady characteristics of the conical vortex on a flat roof on the basis of numerical results by large eddy simulation (LES) when an approaching flow attacks at about 45 degrees against the wall. The results indicate that the large eddy simulation (LES) can give fairly good results, compared with the experiment pressure data of the AIJ flat roof model and other wind tunnel tests. Based on the Q criterion, this paper presented the flow visualization of conical vortices on the roof. By analyzing the mean and fluctuating pressure coefficients on the roof, it is observed that the strong suction acts on near leading edge on the flat roof of the building. The strong suction is caused by a pair of conical vortices whose strength is unbalanced in any time. When the strength of one vortex recedes, the vortex on the other side is strengthened. By adding splitter plates and parapets, the LES results show that the switching of conical vortices may have inseparable relations with the interaction of vortices formed in the wake shed from the sidewalls of the building. Additionally, the parapets can mitigate the high corner suctions on the roof and lift the position of vortex core, while the dimension of the conical vortex is greater with the higher parapets.