Abstract: A high speed circuit is designed for obtaining the BEPCⅡ bunch fiducial signal based on tunnel diode circuit, monostable multivibrator and ECL logic technology. The bunch train with a gap in storage ring is described. The waveforms of the induced signal of the pickup electrode after long cable transmission decay are analyzed. A monostable multivibrator using tunnel diode is described, with which a positive pulse of 0.3 ns is stretched to a 4 ns ECL signal. The ways to find out gap in standard and non-standard injection mode are presented. The test result shows that the circuit works well in standard and non-standard bunch injection modes and the measured timing jitter of 80.49 ps(RMS) fits for the design.