In　order　to　study　the　influence　of　pedestrian　and　bicycle　groups　on　the　semi-actuated　signal　timing　algorithm,　an　analysis　was　made　of　the　characteristics　of　left-turn　vehicles　crossing　pedestrian　and　bicycle　groups　at　two-phase　intersections　first.　Then,　based　on　the　HCM　2000(highway　capacity　manual　2000)　queue　service　time　model,　a　new　model　for　calculating　the　initial　green　signal　time　in　the　semi-actuated　signal　control　algorithm　was　built　using　the　critical　gap　theory　and　the　principle　of　balance　between　the　arriving　and　departing　left-turn　vehicles.　Semi-actuated　signal　timing　plan　was　computed　with　the　proposed　model　using　the　data　collected　in　Huairou　District　of　Beijing,　and　was　validated　by　evaluation　of　the　level　of　service　(LOS)　using　the　index　of　signal　control　delay　in　the　field　experiment.　The　numerical　test　results　show　that　the　total　control　delay　of　the　whole　intersection　decreases　37.8%,　and　the　LOS　improves　from　grade　C　to　grade　B,　so　the　proposed　model　can　make　the　signal　timing　plan　more　adapted　to　intersections　with　many　pedestrian　and　bicycle　groups.