Hi Lingfeng,
Welcome to the TI E2E Forums!
Thanks for the details on your design. I took a look at your configuration settings for the ADS1294R and verified that they were correct. It seems you are saturating the inputs to the device. It's curious to see that the internal test signal is causing a problem, as you have configured the amplitude to equal [1 x -(VREFP-VREFN)/2.4] mV.
Can you measure VREFP-VREFN and tell me the result? Also, are you using dual or single supplies for the ADS1294R? If you are only using single supply, and the test signal amplitude is negative, you will not be able to convert the result and instead the output will always read back "full scale" values.
Regards,