The probability of the last proton being out of the binding potential, P and the root-mean-square (rms) radius (1/2= of the third excited state in 21Na are calculated by means of asymptotic normalization coefficient (ANC)method with the experimental date existing. The results are P=54.73% and 1/2=5.28 fm respectively. The P value and the contribution of the asymptotic part to the 1/2 are compared with those of several other low energy states in 21Na.It is found that the valence particle in the third excited state has an obvious larger P than those of other states. The density distribution of the valence proton in this state is also compared with the nucleons in the nuclear core 20Ne, and a far extended tail can be seen clearly. These results show that the third excited state in 21Na is a proton halo state.