Hello,

You pretty much have two options for this migration:

1) Move the content database from the 2007 farm to the 2010 farm and upgrade the site collection in the content database.  Here is how to do the upgrade of the content database: http://technet.microsoft.com/en-us/library/cc263299.aspx

2) Use third-party migration tools, such as those from AvePoint, Metalogix, and Quest.

Hope this helps.

Thanks for the useful comment. But still I am not clear how can I move the docs from one site collection to second sitecollection.  

You would need to use Migration tools for that if you want to keep their meta data.   If you simply want documents and don't care about the meta data, you can use the Windows Explorer view or even SharePoint Designer to copy them to you desktop and then from your desktop to the other document library.

Please let me know if you know good migration tool.

Metalogix is my personal favorite third-party migration toolset: http://www.metalogix.net/Home/

we don't have budget to buy new 3rd party tool. is 15 day free trial can complete this job?

I am thinking following plan, let me know if it can work,
1. clone MOSS 2007 and upgrade it to 2010
2. detach content DB A and attach it to sql server on farm 2010
3 use "recover data from unattached content database: option to export document library from content DB A to windows folder
4 import document library backup to new SP 2010 using powershell  

thanks.