In the early days of 2017 September, an exceptionally energetic solar active region AR 12673 aroused great interest in the solar physics community. It produced four X class flares, more than 20 coronal mass ejections (CMEs), and an intense geomagnetic storm, for which the peak value of the Dst index reached up to− 142 nT at 2017 September 8 02: 00 UT. In this work, we check the interplanetary and solar source of this intense geomagnetic storm. We find that this geomagnetic storm was mainly caused by a shock-interplanetary CME (ICME) complex structure, which was formed by a shock driven by the 2017 September 6 CME propagating into a previous ICME, which was the interplanetary counterpart of the 2017 September 4 CME. To better understand the role of this structure, we conduct a quantitative analysis on the enhancement of ICME's geoeffectiveness induced by the shock compression. The analysis shows that the shock compression enhanced the intensity of this geomagnetic storm by a factor of two. Without shock compression, there would have been only a moderate geomagnetic storm with a peak Dst value of∼− 79 nT. In addition, the analysis of the proton flux signature inside the shock-ICME complex structure shows that this structure also enhanced the solar energetic particle intensity by a factor of approximately five. These findings illustrate that the shock-ICME complex structure is a very important factor in solar physics study and space weather forecast.