Abstract: Adding reasonable constraint conditions to the observation equation is one of the main methods to solve the ill-posed problem in the GPS water vapor tomography. Various constraint conditions used in the tomography are presented in detail firstly. Then, its effect on the result of the tomography is analyzed based on two simulated GPS networks with different altitude. As the results shown, with the water vapor changing gently in horizontal direction, the weight matrix of horizontal constraints have a large effect on the result of the tomography. In addition, with small altitude differences between stations in the GPS network it will be difficult to detect the water vapor only using the GPS technique, and the solution of the tomography relies highly on the accuracy of a priori information from other water vapor detectors. However, with large altitude difference between stations, the dependence of the tomography solution on a priori information is decreased obviously, and we thus can get a reliable result even without a priori information.