An even iterative predictor for water rocket flight was developed using MATLAB. This proposed predictor can be used to solve the problem of phase II as well as Strutz performed on spreadsheet. Its uses variation of specific mass as the concequence of the compression. Using the initial temperature are 20 derajat C and 25 derajat C in Shotinger model, it needs 0,06 sec to finish the phase II which enhances accuracy of the prediction is 20%. It also shows that the temperature decrease when the fluid in water rocket expanding.