Why does the southwest wind blow in Nanning in summer?

Because Nanning is located on the west side of the anticyclone high.

Not only in Nanning, but also in most parts of China, it is southeast wind in summer, because China is located on the west side of anticyclone high. China is located on the east side of Eurasia, the largest continent, facing the largest ocean and Pacific Ocean. Due to the difference of thermal properties between land and ocean, the continental temperature is high and the air pressure is low in summer, while the ocean temperature is higher than the continental low pressure. The atmosphere moves from high pressure to low pressure, that is, from the southeast ocean high pressure group to the northwest continental low pressure group, that is, the southeast wind is formed.