Abstract: The stability of an external disturbance observer-based double-channel control system would be decreased sharply when plant parameter is varied. Baced on the results from analog computer simulation, the stable parameter region of a double-channel speed control system is discussed, and an engineering estimation method, named "parameter relative perturbation method", is proposed to find the relation between plant parameters variation and system stability. This method can also be used to analyse the effect of plant parameters perturbation on system static error. The parameter adjustment procedure of the disturbance observers is also discussed.