every time when its crash i must restart kerbal space program

What happens exactly? Does it just freeze, or does an error message pop up, or what? Is it crashing every time you launch a rocket with MechJeb?

The first thing to try is to delete your MechJeb installation and redownload and reinstall it. If you are still having this problem, we could have a look at your log file, which is found at KSP_Data\output_log.txt on Windows. Make KSP crash, then stick your log file on mediafire or something and link it here so we can look at it.