A multichannel far-infrared HCN laser interferometer has been developed to measure plasma electron density for the J-TEXT tokamak. Initial operation of a seven chords system with 7 cm spacing and a 0.1 ms temporal resolution is described. Density modulation experiments in J-TEXT show that it is possible to obtain information on particle transport by precise measurement of the density perturbation amplitude and phase spatial variation. An optical configuration upgrade has been implemented in order to improve phase resolution.