A　prestress　force　identification　method　for　externally　prestressed　concrete　uniform　beam　based　on　the　frequency　equation　and　the　measured　frequencies　is　developed.　For　the　purpose　of　the　prestress　force　identification　accuracy,　we　first　look　for　the　appropriate　method　to　solve　the　free　vibration　equation　of　externally　prestressed　concrete　beam　and　then　combine　the　measured　frequencies　with　frequency　equation　to　identify　the　prestress　force.　To　obtain　the　exact　solution　of　the　free　vibration　equation　of　multispan　externally　prestressed　concrete　beam,　an　analytical　model　of　externally　prestressed　concrete　beam　is　set　up　based　on　the　Bernoulli-Euler　beam　theory　and　the　function　relation　between　prestress　variation　and　vibration　displacement　is　built.　The　multispan　externally　prestressed　concrete　beam　is　taken　as　the　multiple　single-span　beams　which　must　meet　the　bending　moment　and　rotation　angle　boundary　conditions,　the　free　vibration　equation　is　solved　using　sublevel　simultaneous　method　and　the　semi-analytical　solution　of　the　free　vibration　equation　which　considered　the　influence　of　prestress　on　section　rigidity　and　beam　length　is　obtained.　Taking　simply　supported　concrete　beam　and　two-span　concrete　beam　with　external　tendons　as　examples,　frequency　function　curves　are　obtained　with　the　measured　frequencies　into　it　and　the　prestress　force　can　be　identified　using　the　abscissa　of　the　crosspoint　of　frequency　functions.　Identification　value　of　the　prestress　force　is　in　good　agreement　with　the　test　results.　The　method　can　accurately　identify　prestress　force　of　externally　prestressed　concrete　beam　and　trace　the　trend　of　effective　prestress　force.