In this paper, ultrashort bunch length monitoring was demonstrated based on Terahertz (THz) coherent transition radiation (CTR) in Tsinghua Thomson scattering X-ray (TTX) source. The radiation produced by electron bunch is split into three paths: one of them is used to detect the total energy, while the other two paths are filtered with different THz band-pass filters before detection. The bunch length variation can be obtained by calculating the ratio between the filtered energy and the total energy. The bunch is compressed by a chicane and via changing the current of chicane, the ratio of filtered energy and total energy changed correspondingly. It is a simple supplemental approach to monitor the bunch length during beam conditioning and facility operation. Bunch arrival-time jitter and nonlinear effects in chicane are observed in the experiment during the measurement of filtered energy and total energy.