Is this Sicilian opening dubious? I have used this opening some times and I won.But sometimes I was almost destroyed.Is the move 6.Qd4 dubious or am I doing something wrong later?   

[FEN ""]

1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 g6 5.Nxc6 bxc6 6.Qd4 Nf6 7.e5 Ng8 8.e6 Nf6 9.exf7
No, 6. Qd4 is not a dubious move. In the variation that you mentioned, the moves that are actually dubious are 7...Ng8 and 8. e6

8. e6 is dubious because it "forces" Black to develop his knight to a good square (!) and also gives up White's strong central pawn on e5, which could have hindered the development of Black's knight on g8. After 9. exf7+, it may seem that Black is robbed of the castling option, but that is just superficial. Black can easily develop his bishop to g7 and bring the h8 rook to e8 or f8 as needed. None of White's pieces can really exploit Black's king position on f7, so White gains nothing from giving up the e-pawn. 

7...Ng8 is dubious because there is a better square for the knight - d5. Also, after 7...Ng8, White isn't forced to play 8. e6, so the knight will have to waste another tempo for development. 

6. Qd4 is playable. It has been played by players of the calibre of former world champion Emmanuel Lasker and also Grandmaster Beliavsky. After 6...Nf6 7. e5 Nd5, the position is equal but playable for both sides. 

 [FEN ""]
 [White "6.Qd4"]
 [Black "Playable"]

 1. e4 c5 2. Nf3 Nc6 3. d4 cxd4 4. Nxd4 g6 5. Nxc6 bxc6 6. Qd4 Nf6 7. e5 Nd5 (
 7... Ng8?! 8. Nd2! (8. e6?! Nf6 9. exf7+ Kxf7) 8... Bg7 9. Nf3 d6 (9... f6 10. e6
 dxe6 11. Bc4 Qxd4 12. Nxd4 e5 13. Nxc6 Bb7 14. Bb5) 10. Bf4) 8. Nc3=