您需要同时merge具有tmp两个临时列的IIUC DataFrames：
import pandas as pd
df1 = pd.DataFrame({'fld1': ['x', 'y'],
'fld2': ['a', 'b1']})
df2 = pd.DataFrame({'fld3': ['y', 'x', 'y'],
'fld4': ['a', 'b1', 'c2']})
print df1
fld1 fld2
0 x a
1 y b1
print df2
fld3 fld4
0 y a
1 x b1
2 y c2
df1['tmp'] = 1
df2['tmp'] = 1
df = pd.merge(df1, df2, on=['tmp'])
df = df.drop('tmp', axis=1)
print df
fld1 fld2 fld3 fld4
0 x a y a
1 x a x b1
2 x a y c2
3 y b1 y a
4 y b1 x b1
5 y b1 y c2
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