Mobile ad-hoc networks are characterized by self-organization, rapid deployment and fault tolerance. A multi-robot formation supported by mobile ad-hoc networks is suitable to some special situations where the communication devices of mobile networks cannot be preinstalled. However, ad-hoc networks are of limited communication range and frequent topology change. If the communication distance of robot isn't taken into account, some robots will escape from the formation whenever it is not within the communication range of the networks. In order to ensure the integrality of the multi-robot formation, a potential field based approach was proposed, in which the special attractive force caused by inter-robot distance, together with the attractive force and the repulsive forces respectively caused by goal and obstacles, has been taken into account. The results in simulation demonstrate that the approach can effectively control the change of multi-robot formation in mobile ad-hoc networks.