The present invention provides wellbore pressure control system and method for well cementation stages, and relates to the offshore oil and gas exploitation field. The wellbore pressure control system comprises: an injection pump; and a control device, configured to control the injection pump to inject a fluid or gas through an injection pipeline to a return pipeline that communicates with an annular space of the wellbore to decrease the pressure in the return pipeline and thereby decrease the pressure in the annular space, wherein, the density of the fluid or gas is lower than the density of a drilling fluid in the annular space. The technical scheme of the present invention can effectively prevent leaky zones from being fractured by high-density cement slurry in the well cementation process that may cause safety accidents such as well kick and well blowout, etc.