Due to the FPGA necessary to evaluate the reliability of clock domain circuit in safe-critical applications. Metastable parameter is a key parameter to influence the clock domain circuits MTBF value. This paper first put forward a step-down method to measure the FPGA devices metastable parameters . Then design the test circuit which use the internal FPGA digital time management module. The experiment result which based on the typical FPGA chip shows that the step-down method can effectively test the FPGA metastable parameterτ and T