I need to show all the picklist value in the formula field....please help

You can create a formula field to concatenate all picklist values into a single string separated by a specified delimiter.

TEXT(IF(ISPICKVAL(My_Picklist_Field__c, "Value1"), "Value1, ", "") &

IF(ISPICKVAL(My_Picklist_Field__c, "Value2"), "Value2, ", "") &

IF(ISPICKVAL(My_Picklist_Field__c, "Value3"), "Value3, ", "") &

...

IF(ISPICKVAL(My_Picklist_Field__c, "ValueN"), "ValueN", ""))