Phenomena of damaged ecosystems and environments have caught the widespread attention of scientists and engineers. Earthquake events are good examples, severely damaging slopes and houses. A study is conducted on slope stability at earthquake limit state so that slopes and relevant houses can be safeguarded. The novel analytical and numerical methods are applied to the study on slope stability at earthquake limit state. The analytical method consults moment equilibrium equations of slopes and a limit equilibrium theory. The numerical method uses the coupled finite elements and strength reduction approach at the earthquake limit state. In addition, earthquake effect for the analytical and numerical methods is considered as a horizontal static force through the pseudo-static method. The validation of the analytical and numerical methods is verified through a comparison between their calculation results. In engineering examples at the earthquake limit state, factors of safety of slopes are shown versus various strength factors of slopes and earthquake intensities. Factors of safety of slopes increase with the increase in strength factors of slopes, including equivalent friction angles and cohesive strength. However, factors of safety of slopes decrease with the increase in earthquake intensities. They imply slopes may be more safe with strong strength factors and low earthquake intensities. These would provide references for the protection of ecosystems and environments.