Correlation is a highly computationally intensive and data-intensive signal processing application that is used heavily in radio astronomy for imaging and other measurements. For example, the next generation radio telescope, Square Kilometer Array Low (SKA-L), needs a correlator that calculates up to 22 million cross products, which is a real-time system with continuous input data rates of 6 terabits per second and equivalent computation of 2 Peta-operations per second. Therefore, a flexible and scalable solution with high performance per watt is very urgent and meaningful. In this work, a flexible FX correlation architecture based on FPGA cluster is proposed, which can be fractal in subsystem level, engine level and calculation module level, simplifying the complexity of data distribution network to increase the system's scalability. The interconnect network between processing engines is a new two-stage solution, using self-developed data redistribution hardware to decouple full bandwidth correlation into several independent sub-bands' computation. And the most intensive calculations, cross-multiplications among all the antennas, are modularly designed under MATLAB Simulink and AMD Xilinx System Generator, which are parametrized to scale to arbitrary antenna numbers with optional parallel granularity to minimize development effort on different FPGA or for different applications. What's more, a fully FPGA-based FX correlator for a large array with 202 antennas, consisting of 26 F Engines based on AMD Xilinx Kintex-7 325T FPGAs, 13 X Engines based on AMD Xilinx Kintex ultrascale KU115 FPGAs, has been deployed in 2022, which is the largest full FPGA-based astronomical correlator as we know.