Abstract: In order to improve the temporal and spatial resolution of high energy electron radiography (HEER), high quality electron beam with low energy spread, low emittance and short pulse length is required for the experiment. In this paper, the electron bunch length was measured by a method based on analysis of the coherent transition radiation energy spectrum. The linac for HEER is based on the thermionic cathode RF gun. The energy spectrum of the terahertz radiation was measured by Michelson interferometer. The electron bunch length was obtained by autocorrelated curve fitting method. The experimental result show that when the macropulse bunch peak current is about 24 mA (the bunch charge about 15 pC), the root mean square (RMS) bunch length is about 0.723 5 ps. Furthermore, the KramersKronig (KK) phase analysis method can be used to reconstruct a possible electron bunch longitudinal distribution. The study of electron bunch length measurement can be used to optimize the electron beam quality and there is great reference significance for subsequent HEER experiments.