使用+:

df.col_2 = df.col_2 + 'new'
print (df)
   col_1 col_2
0      1  anew
1      2  bnew
2      3  cnew
3      4  dnew
4      5  enew

感谢 hooy 提供另一种解决方案:

Thanks hooy for another solution:

df.col_2 += 'new'

或者 assign:

df = df.assign(col_2 = df.col_2 + 'new')
print (df)
   col_1 col_2
0      1  anew
1      2  bnew
2      3  cnew
3      4  dnew
4      5  enew