To address the current problems of low accuracy and poor robustness in solving the vertical deflections using the geodetic level model, a rigorous robust least squares solution for vertical deflections is designed. Firstly, based on the relationship between geoid and vertical deflections, the Earth Gravity Model 2008 is used to calculate the initial parameters. Secondly, this method introduces the median robust method and chooses the Huber weight function to calculate the equivalent weights, and the robust least squares solution of the vertical deviations is obtained by iterating. Finally, the design method is validated by two experimental examples. The results show that the maximum deviation of the vertical deflections components calculated by this method from the theoretical value is around 0.5^", which is more accurate than the comparison method. Meanwhile, the proposed method can effectively resist the influence of gross error and has strong robustness.