The FeCo/Fe3O4 nanocomposite was synthesized using the hydrothermal approach, in which the FeCo alloy and Fe3O4 are formed by one step. The structure of the FeCo/Fe3O4 nanocomposite was characterized by means of Scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray energy-dispersive spectrometer spectroscopy (EDX). They show that the mass ratio of FeCo/Fe3O4 strongly depends on the reaction temperature. Such various architectures follow a stepwise growth mechanism of the composites prepared in various reaction temperatures were also discussed. It indicates that this strategy is facile, effective and controllable for the synthesis of FeCo/Fe3O4 by the one-step method. Furthermore, the magnetic and wave-absorbing properties of the nanocomposites with various structures were investigated in detail. The results show that the FeCo/Fe3O4 with higher mass ratio has higher magnetic properties. Moreover, the FeCo/Fe3O4 nanocomposite shows high wave-absorbing properties (e.g., -37.9 dB), which are expected to apply in microwave absorbing materials.