We searched for rotational lines of H12C14N in comet 46P/Wirtanen, using the Purple Mountain Observatory 13.7 m radio telescope. The J = 1-0 lines of HCN (3.4 mm wavelength) were unambiguously detected in 46P during the 2018 December 14-15 period, with a beam resolution of 62.″2 and a velocity resolution of ∼0.21 km s-1 per channel. The molecular production rates were computed. The mean HCN(1-0) production rate of 46P was (6.45 ± 1.31) × 1024 molec s-1 in the above two days. We found possible indications of time variation in the outgassing activity. The average HCN abundance relative to water Q [HCN]/Q[H2O] of 46P was (0.09 ± 0.01)%. The HCN line shapes gave primary information on the kinematics of the cometary atmosphere. The HCN linewidths provided a direct determination of the coma expansion velocity, which was about 0.5 ± 0.05 km s-1 at post-perihelion (1.056 au).