Abstract: In practical engineering, many rectangular thin-crest measuring weirs use thick concrete walls as the retaining wall of side contracted weirs, leading to the different thickness of the weir and the retaining wall. This change increases the safety of measuring weirs, but different from the standard situation in the specification, and may have a certain impact on overflow capacity. In order to study the calculation of discharge coefficient of rectangular contracted weir considering the thickness of retaining wall, the hydraulic experiments of contracted weir with 6 thickness ratios with contracted ratio of 0.5 are carried out while the nappe over weir is observed and recorded. The experimental results show that the thickness of the retaining wall of the contracted weir has an obvious influence on the discharge coefficient after comparison. Therefore, the calculation formula of the discharge coefficient of the contracted weir in the existing Measurement Specification did not express the influence of the sidewall thickness. Based on the experimental data, the calculation formula of discharge coefficient of contracted weir considering thickness ratio D/d and h/P is established by regression relationship method, which can be used as a reference in practice.