Based　on　the　Euler-Bernoullibeam　theory,　the　dynamical　model　of　the　disordered　two-span　beam　is　established.　The　dynamical　equation　is　solved　by　using　the　modal　superposition　method,　and　the　frequency　equation　of　the　two-span　beam　is　obtained　according　to　the　boundary　conditions.　From　the　frequency　equation,　the　natural　frequencies　of　the　two-span　beam　are　solved,　and　the　time-history　response　curves　of　the　two-span　beam　are　further　obtained.　Considering　the　structural　size　disorder　resulted　from　the　two　different　span　lengths　of　the　two-span　beam,　we　analyzed　the　influences　of　the　different　span　length　ratios　on　the　vibration　characteristics　of　the　structure.　By　comparing　the　results　from　theoretical　calculation　with　those　from　the　commercial　software　Ansys　and　experiments,　good　agreement　is　obtained.　The　research　results　of　this　paper　can　be　of　great　important　on　the　dynamical　analysis　and　design　of　disordered　two-span　beams　in　practical　engineering　fields.　©　2017,　Nanjing　Univ.　of　Aeronautics　an　Astronautics.　All　right　reserved.