Where can I find the file pre_amp.sym，and ths7001_preamp.cir. This two file I can't find on the TI's website
Use the multisim part wizard to make a part.
The spice model is available from TI under the "software" tab on the web page for the part. They call it THS7001_PGA.LIB . Copy and paste this text into the "model" box in multisim.
The symbol can be drawn as a box in multisim with 12 pins. the meaning of the pins can be read from the SPICE model file, in the section called CONNECTIONS (lines with * are comments in spice models).