May you clarify your doubts?
If I understand you correctly, you have the actual minimum thickness of head (assuming corrosion allowance doesn't matter) higher than the minimum calculated thickness, so you are OK. And of course plate before forming must be thicker, as thickness reduces at some locations in the forming process (and increases at other locations).

prex

http://www.xcalcs.com
Online tools for structural design

Adding to prex's post what you have gotten into is the forming allowance for the particular type head you have.
A good point is to tell the head manufacturer the minimum thickness  needed as per design and let him pick the starting plate thickness.  You will nearly always get a little extra thickness and unless you are designing a spaceship you will need it later on.

Checkout the following site as it gives a little in site in to the process and has downloadable spreadsheets for heads.

http://www.pveng.com/Soft/Heads/Heads.htm

Have you reviewed UG-79 and the referenced paragraphs?

See Appendix 3 Thickness of Vessel wall.

Sorry for my mistake. The correct post should be "minimum thickness after forming less than calculated thickness (include corrosion allowance)".

The manufacturer informed me that the minimum thickness of head is before forming. Is this acceptable?

Please advise me.

No, it isn't.
You could start thinking to what type of analysis or test could be used to demonstrate the acceptability of the head, but of course this would cost much more than doing another one (hope it is not already welded to shell, but even in that case...)

prex

http://www.xcalcs.com
Online tools for structural design

Thank you prex. Finally, I find out that the calculated thickness (exclude corrosion allowance) is less than thickness after forming. In this case, it is acceptable.