Abstract: Check dams are widely located in the Loess Plateau area of western China. Their siltation elevation can exceed the design value after long-term running. The excess sediment spreads into the spillway, and then accumulates in front of the overflow weir, leading to the normal flood discharge. This study aims to clarify the influence of siltation thickness on the hydraulic characteristics of the broad-crested weir of check dams. The spillway of a typical check dam was also selected as the research object. The physical model test was finally carried out to analyze the flow pattern, water surface profile, velocity distribution, and discharge coefficient of overflow weir under different siltation heights. The test results were as follows: the upstream water surface of the broad crested weir was straight and flat under the condition of free flow. Then, the water level was dropped near the weir wall and gradually flattened just above the weir crest. Finally, the flow was fallen from the weir to the downstream channel. Sedimentation in front of the weir was reduced the degree of backwater, and thus the water depth dropped significantly. The overall water surface basically remained the same as that without sediment, and then gradually developed towards the open channel flow, with the increase of siltation thickness. More outstanding behavior was also found under small flow or low hydraulic head. The influence of siltation on the weir flow was further quantified using water surface profile. Therefore, the slight decrease was observed on the water surface line in front of and above the weir crest under different cases with the increase of sedimentation height, compared with the water depth along the weir. The water depth in the area behind the weir was depended mainly on the downstream channel, indicating no influence from the siltation height. Furthermore, there was an increase in the flow velocity in front of and behind the weir increases, where the reduced water level indicated the reduction of the cross-section area. Consequently, the rising siltation elevation increased the overflow capacity. It infers that the existing calculation formula cannot be applied for the calculation of spillway design in the check dams with the high siltation height. Thus, it is necessary to quantify the impact of siltation to calculate the discharge coefficient. The test data was substituted into the weir flow formula to deduce the test value of discharge coefficient under various working conditions. The discharge coefficient of the broad crested weir increased with the increase of hydraulic head using the general law of hydraulics. The siltation height shared a great influence on the weir flow coefficient under the same flow condition. Anyway, the discharge coefficient increased gradually with the increase of siltation height, indicating the enhanced overflow capacity of the broad crested weir. Therefore, the fitting empirical formula of discharge coefficient was obtained to introduce the relative siltation height α (Ratio of siltation height to weir height). The average relative error between the calculated and the measured values was 2.7%, fully meeting the accuracy requirements. The new formula can be applied to calculate the discharge coefficient. These findings can provide a strong reference for the check dams reinforcement.