The invention discloses a plane rotation construction technique for a steel truss girder for existing bridge spanning construction. A main girder of the constructed bridge is a steel truss girder spanning the existing bridge and obliquely crossing with the existing bridge; the technique comprises the following steps: firstly, constructing a high-altitude steel truss girder assembling platform which comprises a temporary buttress and a plurality of temporarily assembling brackets; secondly, assembling the steel truss girder at high altitude and removing the temporarily assembling brackets; thirdly, installing a steel truss girder rotation positioning device and a slippage system; before rotating the steel truss steel, firstly installing the steel truss girder rotation positioning device and the slippage system; fourthly, conducting plane rotation on the steel truss girder; and fifthly, dropping the girder in place. The construction technique is simple in steps, convenient to construct, fast to construct, safe and reliable in construction process and less in interference on the existing bridge, and can effectively solve the problems that the construction field is limited, the construction can not be normally conducted, the travelling interference on the existing bridge is large, the construction risk and hidden hazard are large and the like in construction with the existing in-situ falsework method and the dragging method.