When you attach a workbook, please make sure to attach a packaged workbook (.twbx). I was only able to load your workbook because you are using the Superstore Sales data.
I'm confused about your goal. You have a filter on Category, then a filter on Product Name that is selecting the Top 30 products by Sales, using the misnamed "Top Customers" parameter. So this filter on Product Name is always going to return the Top 30 products across whatever dimensions you have in the view.
If you want users to be able to choose the Category, you can right-click on the Category pill on the Filter shelf to turn it into a Quick Filter, and let users select that. You can also add the Sub-Category to the view, and a Quick Filter on that if you like. Users can also always right-click on a dimension in the view and choose to Keep Only or Exclude that.
If you are looking to let the users switch between Top N products by Category or Sub-Category, you can do that by a variety of means, the easiest being a parameter that lets users choose Category or Sub-Category, then a calculated field to return the values for the chosen dimension. I set this up in the attached workbook, with a quick filter on the calculated field so the user can pick what they want to display.
Thanks for your reply, but this didn't reach my purpose
I want to show:
1. Top N prodcuts of all (this is easy to achieve, by set Top N conditon in [product name])
2. Then what is top N products in Furniture or office supplies, or Technology. by the filter setting in step 1, I can only see those <N products which in the list of Top N products, but I want see all N in Furniture. how to set this and is an easy way to switch among categories.....
hope this is clear for you to help
Apologies for only supplying a link, I'm without Tableau this week (don't ask...) so can't do this for you and write it up. This link to the article I'm supplying explains how to show the top N values by category using a set - it appears from your request you want to do something very similar.
Thanks for your help, almost solved!