is my mother can stop my marriage legally?

No Chinnu, in case you and the boy who is going to be your husband are major, your mother cannot stop your marriage legally.

she started giving suicide threat now . if I marry him she will do suicide like .. what can I do now?
shall I go to police to file a compliant that she is giving suicide threat to me ?

Chinnu, you may approch police in order to complaint as to her threats of committing suicide, so that she may be prevented from any such drastic step. I would personally suggest you to take it softly as she is your mother and may be she has some genuine apprehension so it's better to talk to her first with all your family members.b