Finally i rooted my device successfully and it's how i did it ,
This guide is for helping you to root TF300TG with stock rom and this recovery process is only for those devices which have Jelly Bean 4.2.1 stock rom installed and have a unlocked bootloader.
1- Power off the tablet.
2- Download and extract attachment file ( twrp-22.214.171.124-tf300tg-4.2.rar ).
3- Rename extracted file(twrp-126.96.36.199-tf300tg-4.2.blob) to twrp.blob and copy it to your fastboot folder.
4- Connect the tablet to PC via USB cable.
5- Now enter Recovery mode. For this press and hold Volume Down key along with Power key, now press Volume Down key to highlight USB icon or RCK icon.
6- Open command prompt. For this, go to Start. Select ‘Run’ and type ‘cmd’. go to your fastboot folder.
7- type following command-line in cmd-window:
fastboot -i 0x0B05 flash recovery twrp.blob
Remember : Wait a minute until you've seen in the CMD window that the upload is finished.
8- Your device will reboot if not , type following command-line in cmd window to reboot:
fastboot -i 0x0B05 reboot
9- After reboot you just have to keep Volume Down button along with Power button to come back to the selection menu.
in this menu select RCK by volume down and after that you only have to press the Volume Up button, so you get to TWRP mode.
10- In TWRP, tap Reboot, then System and It will ask you if you want to install root so Swipe to install.
11- Once your reboot into the android os follow the SuperSU instructions to complete the root process.
10- Congratulations, now you're just flash the TWRP recovery and enjoy being Rooted.