INA137: Can i using external resistor to In+ and In- to further reduce the gain ?

Hi PYTsai,

As Kai mentions, this is not recommended. Even small mismatch between R126 and R130 in your schematic will significantly degrade CMRR of the INA137. A better solution might be to use a line receiver with buffered inputs, such as the INA1651, and add a "pi" attenuator at the input, as shown:

In this case, because the input impedance of the INA1651 is very high, small variations in R1 and R4 do not have as much impact on CMRR of the circuit. I selected 499Ω and 100Ω resistors to give a 20dB attenuation, approximately the same as your circuit using the INA137, however, these could be increased to 4.99kΩ and 1kΩ to reduce loading on the source.