In high speed PCB design,microstirp lines were used to control the impedance,however,the discontinuous microstrip line can cause signal integrity problems.In this paper,we use the transmission line theory to study the characteristics of microstrip lines.Research results indicate that the discontinuity such as truncation,gap and size change result in the problems such as radiation,reflection,delay and ground bounce.We change the discontinuities to distributed parameter circuits,analysed the steady-state response and transient response and the phase delay.The transient response cause radiation and voltage jump.