Based on the spectrum analysis of the experimental intensive acoustic pulse waveform using FFT, the current research found that the dominating energy of the spectrum is below 100 kHz. It is well known that one of the chemical compositions in sea water, MgSO₄, will cause excessive absorption of acoustic energy due to its chemical relaxation. Starting from a modified equation of state, with special consideration of the relaxation absorption of MgSO₄ in seawater, a new wave propagation equation of intensive acoustic pulse was deduced. Combined with the numerical algorithm, the propagation process of the pulse was simulated and the effects of MgSO₄ relaxation on waveform and spectrum were analyzed. The results indicate that relaxation process will flatten and smooth the steep pulse waveform, and reduce the acoustic energy to low frequency end. The chemical relaxation effects will cause extra loss of the pulse energy, in addition to the geometric spreading effects of pulse wavefront. 