Aim　at　the　deficiency　of　steel　structure　online　stress　monitoring,　a　new　method　of　online　stress　monitoring　based　on　theory　of　nonlinear　acoustoelasticity　guided　ultrasonic　waves　was　proposed.　The　disperse　curves　longitudinal　L　(0,　1)　mode　and　flexural　F　(1,　1)　mode　in　different　stress　level　were　drawn　by　using　acoustoelasticity　motion　equation　and　Pochhammer　frequency　equation.　According　to　the　disperse　curves,　the　optimum　frequencies　of　excitation　wave　were　obtained,　which　are　ranging　from　30　kHz　to　75　kHz,　and　group　velocity　of　L　(0,　1)　mode　and　F　(1,　1)　mode　in　different　stress　level　were　calibrated　for　30　kHz,　45　kHz,　60kHz　and　75　kHz.　The　results　show　that　both　L　(0,　1)　mode　and　F　(1,　1)　mode　guided　wave　group　velocities　decrease　linearly　with　the　increasing　of　stress　in　the　steer　bar.　Acoustoelastic　effect　of　the　above　mentioned　four　frequencies　in　different　stress　level　were　simulated　by　finite　element;　the　results　are　consistent　well　with　the　theoretical　curves,　it　prove　that　stress　level　in　steel　bar　can　be　tested　by　ultrasonic　guided　wave　technique.