how to connect REF_SEL of AD9516-3 when the REFIN is differential pair?

It appears that you want to send in a 2nd reference on the Ref_Sel pin.  This is not possible.  The AD9516 only supports a differential input OR 2 single-ended inputs.  Ref_Sel can be used to manually select the preferred single-ended input.  Ref_Sel is not an additional reference input. 

For multiple differential reference inputs, consider the AD9523 family.

Please clarify your requirement if my assumptions on how you want to use Ref_Sel are not correct.

Thank you for your answer,but you may mistake me.I mean than what should I connect the Ref_Sel if my  reference input is differential as following?

My apologies on the misunderstanding.  I now understand your question.  The answer is that it doesn't matter.  This pin has no effect when the reference receivers are in differential mode.  I'd recommend grounding the pin.  It already has a 30kohm pulldown to ground.

My apologies on the misunderstanding.  I now understand your question.  The answer is that it doesn't matter.  This pin has no effect when the reference receivers are in differential mode.  I'd recommend grounding the pin.  It already has a 30kohm pulldown to ground.