The paper views a relativistic deuteron as a system of two intensively interacting nucleons (a dinucleon approaching) in a formal description of the light cone. The article further deals with the calculation procedure about a mean helicity of a proton in a deuteron within variables of the light cone. This article calls attention to the estimation of a relativistic correction to the mean proton helicity in a deuteron by way of application of available modern realistic deuteron wave functions.