Theoretical electron density sensitive line ratios R1-R6 of SiX soft X-ray emission lines are presented. We found that these line ratios are sensitive to electron density ne, and the ratio R1 is insensitive to electron temperature Te. For reliable determination of the electron density of laboratory and astrophysical plasmas, atomic data, such as electron impact excitation rates, are very important. Our results reveal that the discrepancy of the line ratios from different atomic data calculated with the distorted wave (DW) approximation and the R-matrix method is up to 19 per cent at ne= 2 × 108cm-3. We applied the theoretical intensity ratio R1 to the Low Energy Transmission Grating Spectrometer (LETGS) spectrum of the solar-like star Procyon. By comparing the observed value (1.29) with the theoretical calculation, the derived electron density ne is 2.6 × 108cm-3, which is consistent with that derived from (CV <8.3 × 108cm-3). When the temperature structure of the Procyon corona is taken into account, the derived electron density increases from ne= 2.6 × 108 to 2.8 × 108cm-3.