Spring-mass simulations are widely used in the computer animation of soft objects. Although such systems can be numerically solved by either explicit or implicit methods, it has been difficult to obtain stable results from the explicit methods. This paper presents an easy way to stabilize explicit methods in spring-mass simulations. The simulation procedures are modeled as a linear digital system, and system stability is mathematically defined. This allows us to develop theories of simulation stability. The application of these theories to explicit methods allows them to become as stable as implicit methods.