Abstract: After the establishment of 2000 Chinese Geodetic Coordinate System(CGCS2000),it is one of major jobs to unify various coordinate systems to the CGCS2000.Because the similarity transformation methods applied in the practice for coordinate transformation are in general not so accurate to unify different coordinate systems,an alternative collocation is presented for coordinate unification.It is needed that the prior covariance matrices between the signals and the observations should be consistent with their uncertainties.Otherwise,the coordinate of the stations provided by the collocation transformation will be unreasonable.An adaptive collocation estimator is applied by which the contributions of the signals and the observations to coordinate transformation are balanced.The corresponding adaptive factor is constructed by the ratio of the variance components of the signals and the observations.An estimator of the variance components for the collocation is used.Reasonable and consistent covariance matrices of the signals and the observations are achieved through the adjustment of the adaptive factor.The actual calculation from Xi'an Geodetic Coordinate System 1980(XAS80) to CGCS2000 shows that the accuracy of coordinate transformation is improved by using the adaptive collocation model.