I'm going to create a new mobile UI for opportunity creation page. And I need to reduce some fields on opportunity page in mobile but not in desktop page layout. Is it possible to do that?
Yes it's possible and highly recommended. You can have separate page layouts for mobile and for desktop. Rather than trying to explain here, I'm going to refer you to two different documents for the details that will give you what you'll want need to know.
Between the two, you should have all the resources you need to do what you're asking about, plus much more.
The easiest way of setting a different layout is to use the "Mini Page Layout" which can be customized to limit the number of fields sent to a mobile device from a standard page. You can also create a Flex Page to be used in place of a VF page. Another option is to use browser detection. With browser detection, you can deliver different content to a desktop than to a mobile device. There are multiple solutions to the issue at hand.
This answer applies to the time period before Lightning Experience for desktop became available and is not a valid answer for Salesforce Mobile under LEX.