The mechanism and causes for the surrounding rock deformation of Muzhailing highway tunnel are analyzed using data investigation, theoretical analysis, and field test, and the composite large deformation control techniques are proposed and put into practice to effectively control the large deformation disaster of soft rock tunnel with high geostress. Following are the primary conclusions: (1) the plastic flow of carbonaceous slate and bending deformation of platebeam with thin layer structure caused by high geostress are the main causes of Muzhailing tunnel′s large deformation mechanism, and the main corresponding causes include initial stress field, surrounding rock conditions, groundwater, support concept and system, and construction technology; (2) the traditional timely and robust support are ineffective in areas with severe extrusion deformation, where the support system often needs to be withdrawn and rebuilt; (3) deformation control is achieved through construction optimization techniques such as postconstruction grouting conduit, optimizing steelframe archfoot installation technology, and using anticrack fiber and drain pipe; (4) a composite primary support system is built using the two meanings of tunnel active support theory. According to the field support system test, the composite primary support system has a good major deformation control effect. Finally, the replacement rate of the primary support of inclined shaft No. 2 is reduced from 30% to 0% owing to the construction optimization measures.