Driven by the reanalysis wind data NCEP CFSV2, the third-generation wave model SWAN is utilized in a self-nested grid system to simulate waves in the Beibu Gulf (BG), South China Sea (SCS) for a period of one year. The model accuracy is examined by comparing the numerical results with the Jason-2 satellite altimetry data and the near-shore buoy measurements. Based on the numerical simulations, the influence of spatial resolutions on model predication is evaluated, the seasonal characteristics of waves in the BG are analyzed, and the forcing contributions of local wind in BG and sea waves from SCS are discussed. The results show that: (1) Compared with the Jason-2 satellite data, the root-mean square bias (RMSB) and scatter index (SI) for significant wave height are approximately 0.4 m and 0.2, respectively. Compared with the near-shore buoy observations, the RMSB and SI for significant wave height are about 0.2 m and 0.4, respectively; the RMSB and SI for mean wave period are roughly 0.6 s and 0.2, respectively; and the RMSB for mean wave direction is around 30°. (2) The numerical model with the spatial resolution 12'×12' can predict reasonable results for the open sea area of BG, and the mean relative bias compared to the model results of 2'×2' is not exceeding 10%. (3) In the BG, the northeasterly waves prevail in winter monsoon, the southerly waves reign in summer monsoon, and the southeast waves predominate during the periods of monsoon transition (MT). Waves are stronger in monsoons than MTs, up to the strongest in winter monsoon and down to the weakest when winter goes to summer. (4) The driving contribution of local wind to waves in the BG increases gradually from the bay mouth to the inner bay, and the contribution is stronger in monsoons than MTs. The driving contribution of sea waves from SCS gradually weakens from the bay mouth to the inner bay, and the contribution is weaker during monsoons than MTs. In the middle and northern parts of BG, waves are mainly controlled by local wind. In the water areas to the south and east of Hainan Island, waves are mainly dominated by sea waves from SCS. While in the areas to the southwest of Hainan Island, waves are jointly affected by the both factors.