Emmm, it's so tired to tackle the problem that the blank figure would be save after use plt.savefit(). Primary solution proposed in Google, Baidu is nothing more than calling plt.show() after plt.savefig(fig). However, it doesn't seem to work for my problem. Suddenly, I remember the code that matplotlib.use('agg') I have ever used. After I utilize the aboved code, the brain topographic map is saved as normal!