Magnetization reversal mechanism in perpendicularly exchange-coupled composite hard/soft L10-FePt/CoCrPt bilayers with different soft layer thickness has been studied using magnetometry and magnetotransport measurements. For thin soft layers, the magnetization reversal process can be described by the rigid model. For thick soft layers, a different magnetization reversal process is observed which consists of three stages. An exchange-spring spin structure is first formed from the positive saturation magnetic field to small negative magnetic field. In the second stage, a Néel wall is driven and squeezed. In the last stage, multiple domain structure is formed laterally to reduce the dipolar interaction and the magnetization reversal is accomplished by the pinned domain wall motion in the hard layer.