How to set default value while insert null value into not null column SQL Server?

First Solution,

   insert into t1
    select id,isnull(name,'Peter') from t2

Second solution

ALTER TABLE T1 ALTER COLUMN name varchar(255) NULL

insert into t1
select id,name from t2

ALTER TABLE T1 ALTER COLUMN name varchar(255) NOT NULL

So instead of

Insert into t1 select * from t2

you can rewrite your query as

Insert into t1 
select col1,col2, ISNULL(name, 'Peter'), othercolumns from t2