I want to use libcurl in my android project.How to modify the android.mk to use libcurl?

You can check out the Android.mk for the Tests demo. It links to libcurl.

Basically it's the same as with other makefiles: use -lnameOfLibrary (the "lib"-part is always excluded from the library name) and -LpathToLibrary to get the compiler and linker to find the library.