With the aim of determining the concentration quenching mechanism of Eu2+ doped Ca2BO3Cl, a series of phosphors with a varied Eu2+ concentration (Ca2−xBO3Cl:xEu2+) was synthesized by the solid state reaction method. The phase structure was determined by X-ray diffraction. Photoluminescence (PL) measurements showed broad excitation and emission signatures of the allowed f–d transition of Eu2+ ions. The PL emission intensity was found to be increased by increasing the concentration of Eu2+ ions up to x = 0.03 and then decreased as a result of the concentration quenching effect. The lifetime of the emission from the Eu2+ ions was measured and the decrease in the lifetime with increasing Eu2+ concentration confirmed that non-radiative energy transfer occurred between Eu2+ ions. From the luminescence data, the value of the critical transfer distance was calculated as 1.5 nm and the corresponding concentration quenching mechanism was verified to be a dipole–dipole interaction.