use db
--1.查找出表中该字段的约束名称（或根据已有的提示及对象'DF__******'）
declare @dropconstraint varchar(150) 
declare @conName varchar(50) 
select @conName =b.name from sysobjects b join syscolumns a on b.id = a.cdefault 
where a.id = object_id('dbo.Table_1') and a.name ='aid' 
print @conName
--2. 删除存在的约束
set @dropconstraint = ('alter table dbo.Table_1 drop constraint ' + @conName)
exec (@dropconstraint)