declare @s varchar(max)
set @s='reurl=index.aspx&visitedforums=2;+User_Cookie=UserName=admin&UserPwd=admin;+ASP.NET_SessionId=jph5hebcqk3piitv3vkxj1xg'

select right(PARSENAME(replace(replace(@s,'.','@'),';','.'),2),CHARINDEX(@s,'=')-1)

------解决方案--------------------
SQL code
if object_id('[test1]') is not null drop table [test1]
go
create table [test1]([id] int,[zhang] varchar(92))
insert [test1]
select 1,'eurl=index.aspx&visitedforums=2;+User_Cookie=UserName=admin&UserPwd=admin;+ASP.NET_SessionId'
go

select username=left(stuff(zhang,1,charindex('UserName=',zhang)+8,''),charindex('&',stuff(zhang,1,charindex('UserName=',zhang)+8,''))-1)
from test1

/**
username
-----------------------------------
admin

(1 行受影响)
**/