when put the mouse onto the object,the there is a halo effect around the the object. you couldn’t use a texture with some materia on it ,'cause the object is irregular,when you change the view point the outlook of the object is different .
Some one tell me using shader,I don’t know how to writer such shader.
any help will be appreciate
no need for a shader.
OnMouseOver is the function to use
and the effect you want to achieve can be achieved through creating a dublicate, scale it by -1.01 in each dimension in start and apply the “selection material” to it and bing it will have a halo effect 
just enable / disable this object basing on the onmouseover state
I’ll have a try ~ THX~
while thus the bigger object may shelter the smaller object.
another question is that the halo is surely obscure
“The halo is surely obscure” is a statement, not a question. I don’t know what you mean to ask by it.
Scaling by -1.01 flips the object on all axes. You need to scale by +1.01 and also reverse the normals using the Mesh class.
for ( var n : Vector3 in normals ) {
n = -n;
}