  The weak-field approximation implying linear relationship between Stokes
$V/I$ and longitudinal magnetic field, $B_{\Vert}$, often suffers from
saturation observed in strong magnetic field regions such as sunspot umbra. In
this work, we intend to improve the magnetic field observations carried out by
the \textit{Solar Magnetic Field Telescope} (SMFT) at Huairou Solar Observing
Station, China. We propose using non-linear relationship between Stokes $V/I$
and $B_{\Vert}$ to derive the magnetic field. To determine the form of the
relationship, we perform a cross-calibration of the observed SMFT data and
magnetograms provided by the \textit{Helioseismic and Magnetic Imager} on board
the \textit{Solar Dynamics Observatory}. The algorithm of the magnetic field
derivation is described in details. We show that using non-linear relationship
between Stokes $V/I$ and $B_{\Vert}$ allows us to eliminate magnetic field
saturation inside sunspot umbra. The proposed technique enables one to enhance
the reliability of the SMFT magnetic field data obtained even long before the
space-based instrumentation era, since 1987.
