Abstract: The acoustic system carrier frequency offset (CFO) and impulse noise are the most common interference in the underwater The CFO and impulse noise could be estimated by using the null subcarriers. The null subcarriers estimation included optimal separate estimation and joint estimation. The separate estimation firstly estimated the CFO value and then estimated the impulse noise value. However, the CFO and impulse noise are always affected each other when one of them was estimated. Then a joint optimal estimation was proposed to estimate the both of them. Experimental results performed with numerical simulation and pool-trial data are provided. The results show the improvement of the proposed algorithm under time-varying channels and the joint estimation can get more performance gain margin than the separate estimation about 3 dB at the BER of 10^-2. Through the estimation of them, the robustness is improved.