Wang, Q. Liu, X.Y. Lu, P. Luo, Q. Sun, B.G. Tang, L.L. Wei, J.H. Wu, F.F. Yang, Y.L. Zhou, T.Y. Zhou, Z.R. JACoW Geneva, Switzerland 978-3-95450-182-3 10.18429/JACoW-IPAC2017-MOPAB082 English 309-311 MOPAB082 Contribution to a conference proceedings 2017 2017-05 http://dx.doi.org/10.18429/JACoW-IPAC2017-MOPAB082 http://jacow.org/ipac2017/papers/mopab082.pdf A bunch length monitor using resonant cavity has been designed for the NSRL Infrared Free Electron Laser (IR-FEL) facility. To avoid the restriction of working fre-quency caused by the beam pipe radius, the high order modes of the harmonic cavities are utilized. The position and orientation of coaxial probes are optimized to avoid interference modes which come from the cavity and beam tube according to the analysis formula of electro-magnetic field distribution. Based on the parameters of IR-FEL, a simulation is performed to verify the feasibility of the bunch length monitor. The simulation result shows that the design meets the requirements of IR-FEL, and the resolution can be better than 50 fs.