Abstract: In this paper, different arid environments in the limestone area of Southwest China were simulated by using different concentrations (0%, 5%, 15%, 25%, and 30%) of PEG6000 (polyethylene glycol), and a gradient of exogenous Ca²⁺ (0, 15, 20, 25, and 30 mmol·L^-1 of CaCl₂) was installed to simulate the high calcium soil environments, aimed to study the effects of different concentration PEG6000, CaCl₂, and their interactions on the seed germination characteristics of Koelreuteria bipinnata, one of the preferred species in the ecological restoration in limestone area. Under the action of CaCl₂ alone, medium or low concentration exogenous Ca²⁺ had lesser effect on the seed germination parameters, but high concentration exogenous Ca²⁺ had inhibition effect on the seed germination. Medium or low concentration CaCl₂ could increase the seedling fresh biomass. When the PEG6000 concentration increased from 0% to 25%, the seed germination rate, germination potential, germination index, and vigor index showed a significant decreasing trend, and the seedling fresh biomass decreased remarkably. When certain concentration of exogenous Ca²⁺ was exerted to the drought treatment groups, medium or low concentration CaCl₂ improved the seed germination characteristics, which compensated the loss from drought stress to a certain degree. When the CaCl₂ concentration was up to 30 mmol·L^-1, the seeds were hard to germinate. Additionally, no seeds germinated under the condition of PEG with the concentration of 30%. It was suggested that applying definite concentration of exogenous Ca²⁺ could increase the seedling fresh biomass under PEG6000 stress, and make the seedlings more adapt to arid environment.