Bi-phase modulator is one important component in SLAC energy doubler (SLED) pulse compression systems. Performance parameters such as precision and speed of phase shift have significant impact on SLED. In this paper, an S-band bi-phase modulator with fast speed and adjustable accuracy is presented. Precision of the phase shifter is related to two parallel varactors, controlled by bias voltage. The simulated results show that the proposed bi-phase modulator has a fast switching speed of 4 ns and a high phase shift accuracy.