My back yard flood when it rains,how can I fix this problem?

Talk to a builder or landscaper about grading the dirt away from your yard. You may have to get a professional to do it, or It may be something you can do yourself.

Sounds like you need your yard graded... or depending on the topography around your house (every one else's yard drains into yours) a drain installed to take the water away from your property... Contact a landscape engineer or architect to find out exactly what needs to be done to permanently fix the problem... Check out HomeAdvisor.com or Angieslist.com to find a reputable one....

better to spend money to know... than to spend it several times and not get the problem actually fixed

You may need to install French drains and regrade the yard. I would call in the pros for advice.

This is absolutely a job for the pro's because it could very easily flood your house, and most homeowners insurance won't cover flooding.

you can do a french drain yourself dig 3 feet hole, put in 2 feet of gravel and rock, refill with dirt to top. You can also lay perforated pvc pipe wrapped in yard fabric and put in drain google "french drain"

u need a drainage ditch

Installing a catch basin to stop flooding - YouTube

https://www.youtube.com/watch?v=v21DIhPXOgY

Put in a French Drain or a swale. Directions for both are on u-tube.