Abstract: According to the principle of pulse wavefront compression, the model of a shock line circuit is proposed and the computing formulas of wavefront compression are deduced. The shock line is a nearly continuous high-impedance transmission line loaded at the same small spacings by reverse-biased varactor diodes. With the propagation of the input negative-falltime pulse along the transmission line, the capacitance of varactor diodes is decreased by the increase of reverse bias voltage, and the variation in propagation delay at the nodes results in the compression of negative-going wavefronts during propagation. So the wavefront of ultrafast pulses generated by the shock line can be 1 ps.