I just know how to save as .obj file one by one, but I have a sequence of hundreds of frames of .geo file.

You need to Learn more about Houdini, there are many Good Tutorials for a Beginner.
https://www.sidefx.com/learn/getting_started/ [www.sidefx.com]


For your Question it just need 2 Nodes a File SOP and a ROP Geometry Output inside a Geometry Node.
Made a Example (convert File.hiplc) it requires that your geo Files are a Sequence!
Only then Houdini File SOP can Automaticly read them as Sequence
Sequence Example:
mesh_0001.geo
mesh_0002.geo
mesh_0003.geo
...