Collision-free　locomotion　of　avatar　is　premise　of　its　automatic　constant　navigation　in　CVE.　This　paper　discussed　how　to　design　a　horizontal　collision　detection　scheme　to　prevent　avatar　from　penetrating　into　all　other　virtual　entities　without　stalling　to　achieve　collision-free　motion.　First,　it　proposed　several　design　considerations　for　a　horizontal　collision　detection　scheme.　Second,　it　attached　collision　precaution　and　collision　repulsive　field　to　obstacles　and　divided　process　procedure　of　collision　detection　into　three　processes:　collision　determination,　collision　avoidance　and　path　amendment.　Furthermore,　it　presented　a　new　collision　detection　scheme　PMBHCD　based　on　two　level　2D　projection　maps,　Free　Space　Map　and　Detail　Projection　Map.　PMBHCD　is　discussed　in　three　situations,　applied　different　approaches,　according　to　three　types　of　obstacles:　static　obstacle,　moving　obstacle　and　other　avatars.　Finally,　an　implementation　is　described　in　details.