Abstract: According to the concrete problems of GPR in detecting city roads, the article introduces severa.l technical points of GPR such as central frequency of antenna,laying the measuring line,the measuring line location, testing speed,electromagnetic wave velocity and so on。It is important to access the "first hand" effective information of the target in degree。The article analyses all kinds of factors influence degree to the detecting effect in particular。Thorough practical project,selecting the suitable central frequency of antenna and the best combined parameter can guide the road detecting effectively。The detecting conclusion provides the scientific evidence to repair the road reasonably。