The invention discloses a segment splicing box girder erection and construction technology based on a descending bridge girder erection machine. The descending bridge girder erection machine is adopted for erecting and constructing a constructed box girder. The descending bridge girder erection machine comprises a bottom horizontal supporting system, a main girder system, a longitudinally-moving system, a cantilever crane, a splicing frame and a splicing trolley. When the constructed box girder is erected and constructed, each hole girder of the constructed box girder is erected and constructed from rear to front in the longitudinal bridge direction. The process includes the following steps that firstly, the first hole girder is erected and constructed, wherein a main structure of the bridge girder erection machine is installed, and box girder segments are lifted and spliced; secondly, the next hole girder is erected and constructed, wherein the main structure of the bridge girder erection machine is moved forwards to stride a hole, the cantilever crane is moved forwards, and the box girder segments are lifted and spliced; and thirdly, the second step is repeated till the process of erection and construction of the constructed box girder are finished. By the adoption of the technology, the technological steps are simple, design is reasonable, construction is easy and convenient, the construction process is easy to control, a use effect is good, the box girder erection and construction process can be easily, conveniently and rapidly completed, and the construction quality is easy to guarantee.