I want to use Intel FPGA SDK for OpenCL with Xilinx FPGA.but, Intel FPGA SDK for OpenCL only provides platform refereces of Altera family FPGA. They said user can custom their own FPGA platform, but I'm not sure it is possible to custom my own FPGA platform even if I use non-Altera-family FPGA.
Doubt it! Intel's implementation only supports a subset of its own board, so its highly unlikely that it would support Xilinx as its a competitor! Did you try SDAccel which is the equivalent product for Xilinx? See http://www.xilinx.com/products/design-tools/software-zone/sdaccel.html
No. A custom platform refers to being able to create a board support package (BSP) for an Intel FPGA as an accelerator on your own custom board, as opposed to using a dev kit/accelerator board from Intel FPGA or a partner. Custom platform does not mean you can use X's FPGA with Intel's SDK.