Abstract: This paper presents a new method to measure bunch length of linac with single cavity. Compared with the traditional way, the new method does not need reference cavity. Two eigenmodes of a rectangular cavity are utilized to measure bunch length, so that the improved device is simplified and compact. A single cavity bunch length monitor is designed for the National Synchrotron Radiation Laboratorys Tunable Infrared Laser for Fundamentals of Energy Chemistry. The positions of coaxial probes are optimized to avoid interference modes according to the electromagnetic field distribution. To control the working frequencies of the eigenmodes, the tuning screws are introduced in the cavity, and the method of dimension parameter optimization is put forward. A series of simulations based on CST is performed to verify the feasibility. The simulation results reveal that this monitor meets the requirements of bunch length measurement, and its relative error is less than 5%.