Existing　research　has　been　concentrated　on　improving　the　reliability　of　a　distributed　computing　system　through　optimizing　tasks　allocation,　providing　software　redundancy　and　providing　hardware　redundancy.　None　of　these　works　considered　the　performance　sharing　mechanism　in　a　distributed　computing　system.　Different　from　other　performance　sharing　systems　whose　reliability　can　be　calculated　directly,　the　reliability　evaluation　of　a　distributed　computing　system　with　performance　sharing　is　more　challenging　since　the　reliability　depends　on　the　task　execution　time　of　each　processor　after　performance　sharing.　This　research　considers　a　distributed　computing　system　with　performance　sharing　mechanism　such　that　the　computing　power　can　be　redistributed　among　different　processors　in　the　system.　A　reliability　model　is　proposed　to　evaluate　the　distributed　computing　system　with　performance　sharing.　An　optimization　model　is　formulated　to　derive　the　optimal　performance　sharing　policy　such　that　the　system　reliability　can　be　maximized.　Both　analytic　examples　and　numerical　examples　are　carried　out　to　illustrate　the　proposed　model　and　algorithm.