Abstract: The differences between two groups of survey values for crossing points are only used to check and evaluate the quality of observations in marine survey-line net,but have not been used to correct or adjust errors of observation yet.In this paper,the structure of systematic and random error in marine survey net is discuessed in details and the adjustment method of observations for marine survey net is studied.Based on the survey-line systematic error model,the formula of the rank-defect adjustment model are deduced according to the modern adjustment theory.An example of calculations using real observed data is carried out to demonstrate the effectiveness of this adjustment model.Moreover,the semi-systematic error correction method used now in marine gravity survey in China only is a special case of the adjustment model in this paper.