The decline in groundwater level is a key factor contributing to cover collapse in karst areas. In this study, the model tests and numerical simulations are conducted to reveal the breeding process and formation mechanism of cover collapse sinkholes caused by the decline of groundwater level in karst area. Firstly, the model tests confirm that the decline of groundwater level generates negative pressure at the lower edge of overlying soil. The negative pressure experiences four distinct phases during the groundwater drawdown process: rapid rise, slow decline, rapid decline, and gradual dissipation. The maximum negative pressure is influenced by the particle size distribution of the overlying soil. Then, the numerical simulations are carried out to investigate the change process of negative pressure caused by the loss of fillers in karst pipe. The simulated results indicate that the rate of groundwater decline and the thickness and initial void ratio of the overlying soil can affect the maximum negative pressure. As groundwater level drops, a negative pressure zone forms underground, causing tensile failure in the surrounding soil and creating an arched soil hole, which weakens the support for the overlying soil. This phenomenon can also lead to the collapse of the overlying soil under its self-weight. Groundwater table decline in karst areas can result in both internal and surface collapses. When the overlying soil is thin, internal and surface collapses occur simultaneously. In contrast, for thick overlying soil, internal collapse happens first, followed by a layer-by-layer collapse, ultimately forming sinkholes. Finally, the breeding process and formation mechanism of the Yujiawan Reservoir sinkholes are discussed. Geological conditions and groundwater level decline significantly affect internal collapse in karst areas, requiring careful consideration from on-site engineers.