This　paper　aims　at　analyzing　the　influence　of　non-associated　flow　rule　on　passive　face　instability　for　shallow　shield　tunnels.　The　influence　exerted　by　dilation　angle　on　the　limit　support　pressures,　the　failure　zones　and　partial　failure　ratios　is　investigated　by　numerical　simulations.　Based　on　the　failure　zone　obtained　from　numerical　simulation,　a　log-spiral　mechanism　is　proposed.　The　upper-bound　theorem　is　employed　to　acquire　the　limit　support　pressures,　failure　zones　and　partial　failure　ratios.　The　analytical　results　indicate　that　dilation　angle　has　a　significant　influence　on　the　limit　support　pressure　and　failure　mechanisms.　Finally,　by　comparison　with　the　numerical　models　and　existing　analytical　models,　the　results　indicate　that　the　proposed　log-spiral　model　has　a　certain　accuracy　in　analyzing　the　face　instability　of　blow-out.