A cooperative formation control method based on improved linear extended state observer (ILESO) was proposed for a group of underactuated unmanned vehicles (UUVs) without the speed information from the leader unmanned surface vehicle (USV) during navigation. Firstly, as the speed information of the leader USV was difficult to obtain when the speed sensor failure or communication was disconnected, ILESO was designed to estimate unknown speed information in real time. Secondly, on this basis, by combining cooperative formation error model of multiple unmanned vehicle under Leader-Follower state, the formation control laws were designed by backstepping and Lyapunov theory, and in the same time the lateral speed error and relative yaw angle error were considered as the same subsystem to transfer the lateral force which cannot be compensated directly to the yaw moment which can be directly controlled. Finally, the stability of whole cooperative formation was proved by Lyapunov stability theory. The simulation results demonstrate the stability and effectiveness of the proposed method.