Since the mean reverting is a function in the Hull-White model, this paper develops the estimator and provides the estimation of the mean revering function, which will be estimated by using P-spline estimator and other constant parameters. In addition, we also present an alternative approach to select the regularization parameter. Furthermore, we prove the consistency of the two-stage method and its asymptotic normality of parameters. Finally, based on the zero coupon bond price data, empirical evidences show that there is a slightly less difference between the goodness-of-fit and the constant parameter estimates arising from the bond price less sensitive to short-term rate.