In my level,the terrain is passable,something that is moving can move to where is under the terrain,how can I set its properties?

in wed highlight your terrian and right click goto properties

or write an action for terrian and declare its passable flag off through script

Is the Polygone Flag activated?

cYa Sebastian

The Polygone Flag is not activated,and if I activate it ,the model can aslo go to the place that under the terrain.

Did you change the terrain mesh?
After giving your terrain an action as suggested by badapple, make sure that the mesh is updated after any deformation has been done to it. Include the following code in your action script:

my.passable = off; // in case it's off
my.polygon = on; // in case it's off
c_updatehull(my,0); // updates the hull for all frames

also, make sure the players model is not passable
hope this helps,
cheers

Also , make shure the 'origin' of your model isnt below its feet , and try placing your player high above the terrain (atleast 500 quants).
And , if your player model is too small compared to your terrain and you're using polygon for the terrain , your player sometimes could fall between polygons of the terrain.

next to update_hull, (if you actively manipulate the terrain)

try the following trick:

Move one vertex of the terrain in MED really high, and another one really low.

This way the hight precision in calculation is better.