In order to achieve underactuated stable walking of the biped robot on a discontinuous real ground, a variable step-length stabilized control strategy based on the adaptive feedforward control algorithm is proposed. Firstly, a single-input and single-output adaptive feedforward controller based on CoM (centre of mass) motion state is introduced according to the mapping relationship between robot walking speed, stability and CoM trajectory, to realize stable walking on the real ground. Secondly, a variable step-length stabilized control strategy is proposed from the view of imitating human walking, in order to eliminate the influence of the ground discontinuity on walking stability. The underactuated biped robot can walk stably on the real discontinuous ground by changing the step length and the ideal tracking speed of CoM. Finally, the validity of the variable step-length stabilized control strategy is verified by numerical simulation of the four-link robot model and the prototype experiment on the mixed wood/rubber ground. The experimental results show that the proposed variable step-length strategy can achieve underactuated stable walking on the real discontinuous ground.