What is the advantage of a pre-approved mortgage?

trick question? There is no such thing. There is a pre approval letter, which just says you are likely to get a certain mortgage. As far as mortgage types go, I don't believe there is a pre-approved mortgage type.

i have not seen any preapproved loan saction paper in my life. will you explain it?

the advantages of this type Mortgage Pre-Approval is that the buyer and REALTOR will have confidence in a price range and confidence in obtaining mortgage approval. In submitting offers, sellers will know they have a serious buyer who has taken the time to arrange for mortgage financing first. And just as important, the buyer will be more relaxed in spending money to hire an Attorney for contract review, providing the earnest money deposit.

If several buyers are interested in the same property that you are, being pre-approved can give you a slight advantage. Sellers are more likely to accept an offer from a buyer who has been pre-approved as opposed to a buyer that does not know for sure they can get the amount offered.