From qradar connector all the events fields are not being fetched. Also tried by mentioning the custom event fields in the parameter but didn't worked. need to fetch all the events fields from the offense without custom integration.

Hello ​@Jsp123, thanks for the question.
 
The Custom Fields parameter requires the fields to be entered as a strict comma-separated list that exactly matches the custom fields configured by the user within QRadar (e.g., Field A, Field B).  Assuming you have that correct, the QRadar API might also not return certain fields if they are not properly indexed.  Verify that the missing fields are enabled and indexed in QRadar by going to Admin > Index Management in the QRadar web UI.

Worth noting our docs on QRadar Connectors may prove useful to you as well:

https://docs.cloud.google.com/chronicle/docs/soar/marketplace-integrations/qradar.md#connectors