Using proper beam energy chirp and the undulator detuning effect,we propose a modified optical replica synthesizer scheme to characterize the temporal structure of a relativistic electron bunch,which predicts a 100-fs temporal resolution in numerical simulation.The proof of principle experiment demonstrates a peak current of 9 A and a slice energy spread of about 0.5 keV for the uncompressed electron beam of the Shanghai Deep UV Free Electron Laser Facility.