This study aims to develop a wearable robotic exoskeleton to provide walking assistance for elderly and disabled people with weakened skeletal muscles. Our developed hip exoskeleton is powered by two electrical series elastic actuators covering both hip extension/flexion joints. For exoskeleton control, motion intention perception, delivering appropriate force and flexible human-exoskeleton interaction still remains challenging. To this end, this paper proposes an improved gait synchronizer driven by the only hip angular velocity to synchronize gait phase and further estimate gait features. To adapt to the ever-changing gait, the commanded torque from the nominal biological moment is modulated with estimated walking states and speed. The sEMG acquisition experimental results from 6 healthy subjects demonstrate its effectiveness on movement assistance, i.e., reducing muscle activation efforts significantly for 4Km/h walking and upstairs.