The invention relates to an error reducing method for determining the ratchet wheel boundary of an upper connecting pipe of a pressure container. The error reducing method comprises the following steps: selecting structure parameters related with the ratchet wheel boundary, and constructing at least sixteen models; analyzing each structure parameter to construct at least nineteen explanation parameters; selecting one to four explanation parameters to design at least fourteen different schemes; respectively regressing all the models to obtain predicted values, and selecting a maximum value in a relative residual error as a maximum predicated relative residual error; selecting a minimum corresponding scheme of the maximum predicated relative residual error; adopting a cross check method to establish a regression correlation; utilizing the regression correlation to determine the fitting relative residual error and the predicated relative residual error of each model; determining an optimal regression correlation formula from the fitting relative residual errors and predicated relative residual errors. High precision relational expressions between each coefficient and each structure parameter in fitting functions in the determination of the structural ratchet wheel boundary are obtained at a lower cost, the precision is at least more than 93.58%, and the method is beneficial for the determining of the more suitable ratchet wheel boundary.