Hi Yufang!
The weather at the end of March is typically quite pleasant. You can expect average highs in the upper 70s and low 80s, and average lows in the upper 50s and lower 60s. I would definitely pack sweatshirts or hoodies for the evenings, if you're planning to be out. About 5-7 days out from you vacation, I'd suggest that you check the extended weather forecast at a site like weather.com by searching for the 32830 zip code. It'll give you the best idea of expected conditions for your vacation.
As far as precipitation averages are concerned, March rainfall averages a bit more than January and February, but nothing compared to the rainy season (summer months). We always pack our rain gear, and keep on trucking. A brief rainstorm can very quickly clear out a park, and provide prime time to experience attractions with little to now wait. We just toss a Walt Disney World poncho over our double stroller, and keep on trucking!
If you don't feel like braving the rain, then head to Downtown Disney for shopping, dining, or even bowling at
Splitsville Luxury Lanes! Tons of fun for everyone!
Have a magical vacation!
Amanda