A reflecting FIR interferometer and measurement of electronic density in HL-2A tokamak were presented in this paper. The phase shift was calculated by a new kind of computer software compare technique. The way of self-fitting optical design was used to design the reflector in the chamber. To prevent the first mirror polluted, a special buffer was mount in the vacuum chamber and make the mirror work time longer. The results of measurement electron density show that the interaction between plasma and first wall is lower in diverter discharge than that in limit discharge.