pd.to_datetime用于将您的系列转换为datetime:
s = pd.Series(['2018-01-31 00:00:00'])
s = pd.to_datetime(s)
print(s)
0   2018-01-31
dtype: datetime64[ns]
pd.Series.dt.strftime将您的datetime系列转换为所需格式的字符串:
s = s.dt.strftime('%Y%m%d')
print(s)
0    20180131
dtype: object