Constant fraction discriminator (CFD) is one of theoretic method which can locate timing point at same fraction of echo pulse in pulsed time-of-flight (TOF) laser rangefinding. In this paper, the theory of CFD method was analyzed in reality condition. The design, simulation and printed-circuit-board (PCB) performance of CFD circuit were shown. Finally, an over amplified method was introduced, by which the influence of direct-current (DC) bias error could be reduced. The experimental results showed that timing discriminator could set the timing point to a certain point on echo pulse, which did not depend on the amplitude of echo pulse.