Entry to Bahrain

You would need a visa for Bahrain. And your saudi visa needs to allow for travel via road. It should say open in mode of travel.

While enquiring we came to know that my visa have the permission to travel through the crossway. And stay in Bahrain for around 72 hrs.
Now I want to know about the visa
Can I get the visa while entering or should I take a visa earlier?
Plzz reply me soon sir

I have no idea as you haven't given details.

1) What is your Saudi visa?
2) Do you have a visa for Bahrain?
3) What's your nationality?

I don't have visa for Bahrain.
Am an Indian citizen
And am in visiting visa in ksa

Then you need a visa for Bahrain.   Apply on evisa.gov.bh.

Can I take on arrival visa???

No.  Unless you are an Iqama holder.