这是使用 pd.Series.apply，这只是一个隐蔽的循环.部分字符串合并"就是您要查找的内容，我不确定它是否以矢量化形式存在.

This is one way using pd.Series.apply, which is just a thinly veiled loop. A "partial string merge" is what you are looking for, I'm not sure it exists in a vectorised form.

df4 = df1.copy()

def get_amount(x):
    return df2.loc[df2['Ref'].str.contains(x), 'Amount'].iloc[0]

df4['Amount'] = df4['Invoice'].apply(get_amount)

print(df4)

  Currency Invoice Amount
0      EUR   20561    150
1      EUR   20562    175
2      EUR   20563    160
3      USD   20564    180