The 35-channel Hα (λ = 656.3 nm) line intensity and impurities behaviour of a 9-channel C III, 7-channel bremsstrahlung emission, single channel O II (λ = 441.5 nm) and O V (λ = 278.1 nm) diagnostics were investigated in ac discharges on the HT-7 tokamak. The data analysis identified that the intensity of multi-channel Hα emission and impurities emission of C III (λ = 464.7 nm), O II and O V for the negative current plasma are much lower than for the positive current plasma. Lower hybrid wave plays a different role for the co-driven and anti-driven directions during current transition in ac discharges. Therefore, the emission of C III, O II and Hα behaviours is very different when the current direction transits from positive to negative, and vice versa.