The transition mechanism and “abnormal” temperature effect of 5D 0 - 7F 0 transition of Eu 3+ in EuCl 3·6H 2O are studied. By analysing the intensity ratios of the 5D 0 - 7F 0 transition to the lowest energy level line of the 5D 0 - 7F 2 transition of fluorescence spectra at different tempertures, it is found that 7F 2 state (mainly the lowest energy level) can couple with 7F 0 state by four 245 cm -1 phonons. The 7F 0 state becomes a mixed state, so the “forbidden” 5D 0 - 7F 0 transition can be allowed and its oscillator strength varies with temperature. Therefore, the transition mechanism and the relationship between the oscillator strength of 5D 0 - 7F 0 transition in EuCl 3·6H 2O and temperature are well explained.