This　study　aims　to　solve　the　difficulties　in　measuring　the　speed　of　rainwater　in　pipes.　On　the　basis　of　the　analysis　about　tracer　particles　size　and　number　distribution　in　rainwater　pipes　and　tracer　particles　velocity　distribution,　Laser　Doppler　in　Velocity　(LDV)　was　used　in　this　paper　to　measure　the　speed　of　solid　impurities　within　the　rainwater　pipe,　which　are　regarded　as　the　tracer　particles　that　represent　the　speed　of　rainwater.　In　order　to　verify　the　feasibility　of　the　LDV　method　in　measuring　the　tracer　particles　in　rainwater,　a　novel　mathematic　model　of　LDV,　which　is　based　on　the　theoretic　model　of　fringes　and　the　principle　of　light　interfering　and　scattering,　has　been　developed　to　simulate　and　analyze　the　relationship　among　the　characteristics　of　the　tracer　particles,　the　velocity　of　rainwater　and　the　accuracy　of　the　velocity.　Analog　signals　were　obtained　after　simulation.　The　CHRONUX　analysis　and　time-frequency　analysis　were　used　to　find　the　frequency　of　the　analog　signals,　and　then　convert　the　frequency　into　the　speed　of　the　rainwater　in　pipes.　The　results　show　that　the　method　proposed　in　this　study　has　relatively　high　accuracy,　which　is　better　than　other　methods　of　velocimeters.　This　study　indicates　that　the　LDV　can　apply　to　the　speed　measurement　of　rainwater　in　practice.　©　2012　Huazhong　Univ　of　Sci　&　Tec.