The invention discloses a steel truss girder assembling, erecting and construction process. A steel truss girder is a girder body, the front end and the rear end of an erected steel truss girder are supported on a first permanent buttress and a second permanent buttress respectively, the steel truss girder is formed by splicing a plurality of steel truss girder segments in an end-to-end mode, and the first permanent buttress and the second permanent buttress are located on the two sides of a main riverway of a river respectively. Assembling and erecting of the steel truss girder include the first step of construction of temporary supporting systems, longitudinal sliding rails and transverse sliding rails, the second step of assembling, erecting and longitudinal moving of the steel truss girder at different times, the third step of transverse moving of the steel truss girder and the fourth step of setting of the girder, wherein in the first step, the temporary supporting systems are constructed on one side of the first permanent buttress and on one side of the second permanent buttress respectively, the two longitudinal sliding rails used for the steel truss girder to move longitudinally and the two transverse sliding rails used for the steel truss girder to move transversely are distributed on the temporary supporting systems; in the second step, the steel truss girder segments are assembled more than once from front to back and are longitudinally moved many times. The steel truss girder assembling, erecting and construction process is simple in process, reasonable in design, convenient to achieve, good in construction effect and little in construction difficulty, and the assembling and erecting processes of the steel truss girder can be conveniently and quickly achieved.