The study of MHD waves in coronal structures is of great importance in coronal seismology. The study of these waves makes it possible to reveal the physical structure and heating mechanism of the solar corona. It is of great interest to calculate the line profile in the emission spectrum of a magneto-sonic wave for various physical parameters, calculate the energy flux and compare them with observations. In this paper, the profiles of the Fe IX 171  line in the emission spectrum of slow magneto-acoustic waves propagating in coronal loops are calculated for cases of an optically thin layer and the change in density. The line profiles were calculated for the following parameter values: wave velocity amplitude 10 km/s, coronal loop width 2000 km and 5000 km, wavelength = 20000 km and 50000 km, Doppler width = 0.01 , and at values of the angle of the line of sight and at different phases of the wave. The energy flux density is 622.5 W/m². The calculated values of the energy flux density strongly depend on the angle of the line of sight and on the phase of the wave and range from zero at large values of to ~4 W/m², the values of Doppler velocities and velocities of non-thermal movements at small values of have a maximum value of ~13 km/s and decrease almost to zero at large values of . At different values of the angle of the line of sight, the asymmetry is almost not noticeable. An interesting result is that the values of the calculated (observed) energy flux can be both much less and much more than the true value: from almost zero at small values of . These values depend not only on the angle of the line of sight, but also on the width of the coronal loop and the wavelength.