I used navmesh graph and my source mesh may have over 40000 verts and 10000 tris. This may cause a lot of memory and how to solve it?
10000 tris is not that much memory really. It will use maybe a few megabytes depending on your settings, but I wouldn’t worry about it too much.
The system requires some memory for every node and thread that is used. So reducing the number of pathfinding threads can reduce the memory usage.
Otherwise there is not really that much that can be done in this case (without modifying the source code heavily). There are some options for other graphs types, but not for pure navmesh graphs.