The low angle tracking problem was investigated for shipborne X-band array radar working at frequency diversity mode. Firstly, the receiving array signal model with multi-frequency was established in multi-path environment based on the relation between complex reflection coefficient (CRC) and grazing angle of seawater at X band. Then, the research put forward the MFML (Multi-Frequency Maximal Likelihood) elevation spectrum, in which the elevation ambiguity is alleviated by frequency diversity. The spacing between spectrum peaks was presented. The influence on elevation spectrum of frequency width was analyzed. Lastly, the probability of exact peak selection was proposed to describe the elevation estimation performance. Elevation estimation performance was evaluated by simulation, related to SNR and target elevation. Given target elevation, the minimal SNR was obtained for a certain angular accuracy, which is not strictly monotone downtrend with target elevation.