Abstract: Base on the related formulas of JGJ 120-99 technical specification, with steady flow theory, tube wells are designed for dewatering in a foundation pit locate in phreatic aquifer close to fiver. Through one practical engineering, the applicability and the limitation of the formulas in JGJ 120-99 technical specification about the single well yield and the ground water drown down in one point of the foundation pit is discussed, and the improvement and perfection of the corresponding formulas suit for this state is put forward.