We optimized synthesis conditions of blue-emitting CaMgSi2O6:Eu2+, (CMS:Eu2+) with conventional solid-state reaction and successfully determined structure parameters by Rietveld refinement method with neutron powder diffraction data. The final weighted R-factor R-wp was 6.42% and the goodness-of-fit indicator S (= R-wp/R-e) was 1.34. The refined lattice parameters of CMS:Eu2+ were a = 9.7472(3) angstrom, b = 8.9394(2) angstrom, and c = 5.2484(1) angstrom. The beta angle was 105.87(1)degrees. The concentration quenching process was observed, and the critical quenching concentration of Eu2+ in CMS:Eu2+ was about 0.01 mol and critical transfer distance was calculated as 12 angstrom. With the help of the Rietveld refinement and Dexter theory, the critical transfer distance was also calculated as 27 angstrom. In addition, the dominant multipolar interaction of CMS:Eu2+ was investigated from the relationship between the emission intensity per activator concentration and activator concentration. The dipole-dipole interaction was a dominant energy transfer mechanism of electric multipolar character of CMS:Eu2+.