I developed a project in flex and catalyst and the size of swf is too much, sometimes its take too much time to load. Product is already ready, so i cant change in code too much. Please help me to solve this issue. one more thing sometimes its load very fast and sometimes its take lots of time. Why its like that?
Quick answer is that there's no quick solution. There are several things you can do like caching RSLs (using the Adobe signed one), creating your own RSLs, dividing your application into modules, remove all embedded media and make it load on demand, use vectors (FXG) instead of bitmap files (png, jpg, etc) and make sure you don't add things that aren't needed.
All of these can make an application load faster and be much more responsive. The problem is that it's not a quick solution and needs quite a bit of architecting. Normally, you start this from the beginning of your project. From the sounds of it, you didn't know most of these concepts and you've added everything to one massive swf file (including RSLs).
You've got a long road ahead of you.