Firstly, many thanks for you detailed reply.
When ADIS16448 is powered and keep the default setting unchanged, the values that i read from SMPL_PRD and SENS_AVG are 0x0B01 and 0x0402.
I restore SMPL_PRD to 0x0001(WriteToADIS16448(0xB601)), but the value of SMPL_PRD changes to 0x0200 and SENS_AVG's changes to 0x0000.
I bought two ADIS16448, they have the same problem. One, i can accept; two, i can't.