Abstract: This paper proposes an improved bunch length measurement method based on high order mode cavity. Different from traditional ways, the new method uses the cavity that resonates with higher order mode TM0n0 instead of fundamental mode TM010. The operating mode of this new method is alterable, which means that harmonic cavities with larger radius are available and working frequency restriction caused by beam pipe radius is removed. A two-cavity monitor was designed for the linac of future National Synchrotron Radiation Laboratory(NSRL) positron source. Operating frequency selection is discussed and calculation formula of bunch length is derived. The bunch length measurement cavity resonates with mode TM020 at 14.28 GHz, which provides much larger radius than TM010 mode. A simulation measurement was conducted in CST Particle Studio and the result shows a fairly high accuracy (better than 7%).