The synergistic effect of total ionizing dose (TID) on single event effect (SEE) in SiGe heterojunction bipolar transistor (HBT) is investigated in a series of experiments. The SiGe HBTs after being exposed to Co-60 gamma irradiation are struck by pulsed laser to simulate SEE. The SEE transient currents and collected charges of the un-irradiated device are compared with those of the devices which are irradiated at high and low dose rate with various biases. The results show that the SEE damage to un-irradiated device is more serious than that to irradiated SiGe HBT at a low applied voltage of laser test. In addition, the gamma irradiations at forward and all-grounded bias have an obvious influence on SEE in the SiGe HBT, but the synergistic effect after cutting off the gamma irradiation is not significant. The influence of positive oxide-trap charges induced by TID on the distortion of electric field in SEE is the major factor of the synergistic effect. Moreover, the recombination of interface traps also plays a role in charge collection.