Oracle is one of the most widely-used large-scale databases at present. For its complex architecture, Oracle's performance is subjected to various factors, and query operation is one of the key factors. This paper analyzes the sharing principle and execution process of SQL statement in Oracle database. It finds out that improving the sharing of SQL statements and the data cache hit rate of the SQL statement is the premise of performance tuning. Lastly, it presents four principles for SQL performance tuning.