Abstract: Solving the problem of large data capacity is a major challenge in SOC test. In this paper, a kind of filling way is presented,which based on don't-care-bit of varible-run-length coding test data compression scheme.The scheme used variable-to-variable run-length coding way to compress the data. Theoretical analysis and experimental results show that the X-filling method can obtain high compression rate without any increase in test power. A decompressiom structure based on FSM is proposed at the same time.