Current I have to use two softwares for poster designing.
The text I have to write in corel draw and then I export it in eps and then use Photoshop to give some text effects like bevel shadow etc.
If I write text in Photoshop then that text is very low quality
but in Corel Draw I can't give text effects.
Is there any one software I can use
Corel and PhotoShop should work fine. Depending on the poster you'd likely be using a mix of vector illustration (Adobe Illustrator, InkScape, Corel Draw, etc.) and Raster image editing (PhotoShop, GIMP, etc.) and maybe some page layout software as well (QuarkXpress, InDesign, Scribus).
That said, you normally wouldn't want a raster image editor to be your final file format as you found out, you'll have trouble making crisp type unless you are using a really high end image for it all. For text effects you can only make in PhotoShop, I'd consider doing that in PhotoShop, but then import that back into Corel as an image so you can finish up the rest of the text.