A　state　space　description　with　two　inputs　two　outputs　is　given　to　express　the　PID　control　system.　The　description　has　the　same　control　structure　and　signal　transfer　relations　with　the　PID　control　system　and　avoids　the　approximations　and　redundancies　of　existing　methods.　Upon　this　description,　PID　parameters　can　be　tuned　by　state　space　methods　such　as　tracking　control,　optimal　control,　etc.　Then　the　PID　optimal　controller　based　LR　is　discussed.　Some　samples　are　given　to　verify　the　effectiveness　of　our　method.　The　method　given　here　is　more　flexible,　more　utility　than　existing　methods,　and　can　be　used　on　different　degree　systems.　©　2018,　Editorial　Department　of　Control　Theory　&　Applications　South　China　University　of　Technology.　All　right　reserved.