Abstract: Based on the hypothesis that phosphorus nutrient variation consists of linear trend and periodical fluctuation , a model of phosphorus nutrient variation (PVM)is developed .The stepwise choice method is adopted to solve the model parameters ;such as period parameters based on observed data of unequal time interval series .The satisfactory results have been obtained by using the PVM model in fitting the observed data of phosphorous nutrient and verifying the phosphorus nutrient variation as a forecast measure .It is shown that the fluctuation due to the periodical components has much more effect on the variation of phosphorus in the sea than the corresponding trend component .It is also showed that no periodical component has the dominant effect in phosphorus variation and it is the combined effect of periodical phenomenon in several time-scales to determine the variation of phosphorus nutrient .