While many challenges remain with respect to the mechanical design of the lower limb exoskeleton, it is equally challenging and important to develop effective control strategies. The exoskeleton is a highly human-robot coupled system with a complex dynamic model and working environment, so it is crucial that the controller works in concert with the user intention without relying on imprecise models. This paper proposes a motion controller for a lower limb exoskeleton, aiming to perform collaborative squatting assistance with efficiency and flexibility. This control strategy is designed for our exoskeleton which is equipped with a force sensor on the back. The high-level control is a force-velocity admittance model estimating the human intention by the interaction force, and the low-level control is based on PD closed-loop velocity control with gravity compensation. Through experimental studies conducted with our exoskeleton, the feasibility and effectiveness of the control strategy are demonstrated.