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@Mona Song wrote:

How should I explain to my client why they still need to pay ES payment when they don't owe taxes after the total tax is lesser than total payment. 

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They don't really need to make the Estimated Tax payments. 

BUT you should explain that *IF* there were to owe tax next year, their high income would eliminate one of the 'exceptions' to the penalty, so they may consider making additional tax payments to avoid that possibility.  Estimated Tax payments are one way of doing that, but increasing withholding from their job is usually an easier way to do it (other than figuring out how to fill out the W-4). 

But underpayment of tax penalty would only apply (1) if they owe tax next year, and (2) they don't meet any of the other exceptions to avoiding that penalty.  So if they aren't concerned about it, they can forget about it.