The synergism effect of total ionizing dose (TID) on a single event transient (SET) in a bipolar comparator is investigated. Experimental results show that the shapes of the SET are considerably influenced by the TID accumulated in low dose rates. The variation tendency of SET shapes can be accurately simulated by temperature switching irradiation. The mechanism of this synergism effect is also analyzed in brief via the operating schematic of a comparator. After the accumulation of 100 krad(Si), the lower tendency of negative SET can be attributed to the degeneration of beta. The change tendency of a positive SET, either lower or higher, is dependent on the load condition that limits the output range of the comparator.