Influence of the intermediate Au layer during annealing in hydrogen ambience on the hard magnetic, chemically ordered $L1_0$-FePt phase formation in film Fe$_{50}$Pt$_{50}$(15 nm)/Au(7.5 nm; 30 nm)/Fe$_{50}$Pt$_{50}$(15 nm) compositions deposited by the magnetron sputtering on SiO$_2$(100 nm)/Si(001) substrate is investigated. The annealing of films is carried out in hydrogen ambience within the temperature range of 500–800°C for 30 s. The rates of heating and cooling are of 1°C/s. Before gas inflow, the chamber is pre-pumped out to a pressure of 0.1 Pa. The hydrogen pressure at annealing is of 101.3 kPa. The $A1$-FePt $\to$ $L1_0$-FePt phase transition begins during annealing at 500°C in both films. The ordering degree of the $L1_0$-FePt phase, coercivity $H_C$, and sizes of FePt grains are increased, when annealing temperature is elevated. During interdiffusion of the Au and FePt layers, gold does not participate in the ordering process of FePt. It is located in the intergranular area that limits the exchange interaction between the $L1_0$-FePt phase grains and additionally increases $H_C$. Moreover, coercivity additionally rises with Au layer thickness increasing. Introduction of hydrogen atoms into the octahedral interstices of the $L1_0$-FePt crystal lattice leads to a decrease in the tetragonality degree by means of the increasing the lattice parameter $c$ during annealing at temperature above 600°C. The penetration of hydrogen into the Au lattice leads to gold-hydride formation. The preferred (111) texture is formed in both Au and FePt layers.