Abstract: This paper proposed the kinematic model and load-bearing mechanics model of the four-leg support based on the plane bar system to tackle the unclear formation mechanism of the non-ideal posture of the hydraulic support in the setting stage under the condition of soft surrounding rock.Then, this study obtained the velocity relationship between the front and rear leg when the canopy moving in a fixed posture at the setting stage, as well as the variation characteristics of the resultant force and resultant force positions of the canopy and the base.The results of theoretical analysis and numerical calculation show that only the setting stage is the best time to adjust the posture of the support.When the canopy rises in a horizontal or head-up posture compared with the base, the velocity ratio of the front and rear leg is a cubic function.When the canopy rises in a head-down posture compared with the base, the velocity ratio of the front and the rear leg shows a five-order function relationship.The change of the resultant force positions of the canopy and the base depends on the posture of the canopy contact to the roof, which tends to change fast at first and then slow.The increase of the resultant force of the canopy and the base is slow at first and then rapid.The research results can offer reference for the design of support structure and posture controller, as well as field operation.