Voluntary cancellation of gst registration

yes, u can apply voluntarily for cancellation of GST registration. Cancellation of registration option is available at GST portal.

what all requirement for same also which gstr to be filed

There are the following reasons for cancellations:
1. Change in constitution I.e. Conversion of proprietor into partnership or in other form and vice-versa.

2. closure of business.

3. sale of business, merger, amalgamation etc.

4. Death of proprietor
or any other reason.

As such there is no specific requirement for cancellation provided u paid earlier proper taxes.

Option for cancellations of registration is available under Service tab after GST login.

if initially i have taken registration but afterward If my turnover is less than 20 lac can i cancel

yes dear you can apply for voluntarily cancellation of registration even your turnover doesn't exceed 20 lakhs rupees.
on the above ground, u can apply for cancellation provided u have complied all the provision.

You have to file all your pending Returns ( if any ) and clear off all Liabilities with the GST Department before proceeding for Cancellation of your Registration Number.

Refer SECTION 29 of CGST Act 2017 for more information.

if registration is cancelled than it is compulsory to file annual return or not.?