This work justifies the nonlinearity errors in ionic electroactive polymer actuator lumped-parameter models. The errors are traced back into the model parameters variations as functions of voltage and payload mass. Six experiments with different voltages and payload masses were carried out. Experiments showed that a linear lumped-parameter model with corrected parameters accurately predicts actuator's behaviors as a linear second-order system. Therefore, nonlinearities are mapped to lumped parameter variations. The changes in resistance, capacitance, mechanical pole, and mechanical gain according to the voltage and payload mass are calculated and illustrated. Mechanical pole and mechanical gain are functions of mechanical parameters, namely, second moment of inertia, Young's modulus, and viscosity. Geometrics and electrical transitions due to actuator movement and voltage are responsible for the changes in parameters. The results are promising for a linear and straightforward but accurate loaded and unloaded actuator model.