Hybrid soft leg systems have been studied for advanced gaits of soft robots. However, it is challenging to analyze and control hybrid soft legs due to their nonlinearity. In this study, we adopted dynamic pole motion (DPM) to analyze stability of a nonlinear hybrid soft leg system with dynamic Routh’s stability criterion and to design a proper controller for the nonlinear system with an error-based adaptive controller (E-BAC). A typical hybrid soft leg system was taken as an example, as such a system can easily become unstable and needs a controller to get the system back to a stable state. Specifically, E-BAC was designed to control the unstable hybrid soft leg fast with a minimal overshoot. As a nonlinear controller, the implanted E-BAC in a feedback control system includes two dominant dynamic parameters: the dynamic position feedback $K_p\left(e,t\right)$ and the dynamic velocity feedback $K_v\left(e,t\right)$. These parameters were properly selected, and the feedback was continuously varying as a function of system error $e\left(t\right)$, exhibiting an adaptive control behavior. The simulation shows that this approach for constructing an adaptive controller can yield a very fast response with no overshoot.