运行这个：

SELECT CASE transaction_isolation_level 
    WHEN 0 THEN 'Unspecified' 
    WHEN 1 THEN 'ReadUncommitted' 
    WHEN 2 THEN 'ReadCommitted' 
    WHEN 3 THEN 'Repeatable' 
    WHEN 4 THEN 'Serializable' 
    WHEN 5 THEN 'Snapshot' END AS TRANSACTION_ISOLATION_LEVEL 
FROM sys.dm_exec_sessions 
where session_id = @@SPID

常量值的 docs.microsoft.com 参考。