Abstract: Active phased array antenna is made up of multi-channels. Noise-level is closely related to noise figure of active phased array antenna, noise-level is magnified by high gain channels and then radar detection range is deduced by noise-level. In this article, combined with link characteristics of phased array antenna with various lengths, noise figure of parallel multi-channels of phased array antenna is analyzed in detail and deduced theoretically, this provides a theory guide for chain design in phased array antenna. An applied example is given to verify the accuracy of the formula, which shows that our design has directive meaning to engineering project.