This paper studied the cross-domain fixed-time formation control problem of an air-sea heterogeneous unmanned system (ASHS). Taking advantage of individual characteristics of unmanned aerial vehicles (UAVs), unmanned surface vehicles (USVs), and unmanned underwater vehicles (UUVs), the ASHS was introduced to accomplish the ocean stereoscopic observation mission, which was transformed into the formation control problem. Due to the difference of dimension and the communication constraint between UAV, USV, and UUV, a cross-domain communication protocol was proposed to achieve the state information flow between heterogeneous unmanned vehicles with different dimensions rather than construct the identical low-dimensional output. The high-dimensional unmanned vehicle can receive full state information from low-dimensional neighbors, while the low-dimensional follower can only receive partial state information from high-dimensional neighbors. Moreover, by means of fixed-time control theory and the backstepping control technique, the distributed fixed-time observer and controller were designed to solve the formation control problem for the ASHS with disturbances. Simulation results show that the ASHS can achieve fixed-time formation control with the proposed coordinated control protocols. Meanwhile, the convergence time of the proposed fixed-time formation control protocols is independent of the initial state.