In order to improve the mechanization level of arch frame construction of primary support in the early stage of fullface and microbench excavation of large crosssection weak surrounding rock tunnel, the threeboom arch mounting trolley is introduced. The parameters, properties and construction technology of the theeboom arch mounting trolley are analyzed, and comparison of applicable effect between the trolley and the traditional one is made. Some conclusions are drawn as follows: (1) The grapping and connecting arches of the trolley can effectively shorten the arch mounting time without hurting operates. (2) The steel arch installation process is more standardized, which can effectively improve the installation accuracy and quality of the arch. (3) Comparing to the traditional arch mounting equipments, the threeboom arch mounting trolley is more suitable for the arch frame construction of primary support of large crosssection soft surrounding rock tunnel by microbench and fullface excavation methods. (4) The trolley can also assist in the construction of some tunnels at high altitude.