Recently,　several　techniques　have　been　proposed　to　protect　the　user　location　privacy　for　location-based　services　in　road　network　environments.　A　typical　approach　for　user　location　privacy　protection　is　to　blur　a　user　location　into　a　cloaked　set　of　road　segments　S　such　that　S　satisfies　the　user　specified　privacy　requirements.　However,　these　location　cloaking　algorithms　work　with　snapshot　locations　only　and　do　not　consider　the　effect　of　continuous　location　updates.　Applying　these　algorithms　directly　to　continuous　queries　would　lead　to　privacy　leakage　if　attackers　have　knowledge　about　maximum　user　velocity,　namely　velocity-based　Attack.　In　this　paper,　we　present　the　privacy　safety　condition　to　defend　against　velocity-based　attack,　and　propose　an　anonymity　algorithm　based　on　greedy　strategy　to　preserve　user　privacy.　The　experiment　results　based　on　dataset　of　real　network　show　the　algorithm　is　effective　and　feasible.　©　2012　IEEE.