Recently obtained iron-based compounds (ferropnictides and ferrochalcogenides) can be easily considered as high-Tc superconducting materials. These materials have a wide range of magnetic states [1]. In order to describe the magnetism of FeAs-based compounds we consider the following issues: interaction exchange between localized moments, nesting on the Fermi surface, restructuration of electronic energy spectrum. Due to this fact a magnetic state related to spin density wave (SDW) should be considered. Starting from the Hubbard Hamiltonian in mean-field approximation, we obtain the relations for the magnetization parameter MQ, where Q is the wave vector for SDW, and the spontaneous magnetization M. Further, a chain of recurrent equations for Green functions is deduced. Replacing the solutions for the system of equations for Green functions in the definition of the values MQ and Q, we obtain the system of equations for order parameters for the case of an external magnetic field arbitrarily oriented to magnetization. The obtained system is transformed taking into account the Uprocesses. The particular cases when the external magnetic field is parallel and perpendicular to magnetization are analyzed and the conditions for the transition of the system to the commensurable and incommensurable SDW state are examined. The magnetic transition temperature is suppressed strongly in longitudinal magnetic field and weakly in the transversal one [2]. The dependence of magnetic transition temperature TM on impurity concentration x is obtained. In such a way the phase diagram (T, x) is possible to build. This diagram contains commensurable and incommensurable state of SDW. The SDW is stabilized by the incommensurable state.