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Context. The Si IV lines at 1394 \r{A} and 1403 \r{A} form in the solaratmosphere at a temperature of $\sim10^{4.8}$ K. They are usually consideredoptically thin, but their opacity can be enhanced during solar flares.Traditionally, the intensity ratio of these lines are used as an indicator ofthe optical thickness. However, observations have shown a wavelength-dependentintensity ratio profile $r(\Delta\lambda)$ of the the 1394 \r{A} to 1403 \r{A}lines.  Aims. We aim to study the variation of the intensity ratio profile in solarflares and the physical reasons behind it.  Method. The Si IV lines and their intensity ratio profiles are calculatedfrom the one-dimensional radiative hydrodynamics flare model with non-thermalelectron heating.  Result. During flares, $r(\Delta\lambda)$ is smaller than 2 at the line corebut larger than 2 at the line wings. We attribute the deviation of the ratiofrom 2 to two effects: the resonance scattering effect and the opacity effect.Resonance scattering increases the population ratio of the upper levels of thetwo lines, and as a result, increases $r(\Delta\lambda)$, in all wavelengths.The opacity effect decreases $r(\Delta\lambda)$, especially at the line corewhere the opacity is larger. These two effects compete with each other andcause the U-shape of $r(\Delta\lambda)$.

On the intensity ratio variation of the Si IV 1394/1403 \r{A} lines during solar flares