The commonly used destructive testing of traditional railways ballast track is not suitable for high-speed railway ballast-less track. At present, high-speed railway subgrade status detection mostly depends on the non-destructive method of track inspection car, but the weakness is that the track quality index (TQI) only represents the geometric state of the track, which can't directly reflect the subgrade service status or the severity of the subgrade defects. In view of this, with the Shanghai-Nanjing intercity high-speed railway as the study object, the rail inspection car detection and post-construction settlement observation were conducted to carry out the study on the service status monitoring of high-speed railway subgrade. On the basis of TQI statistical diagram and post-construction settlement curve of the whole line, the correlation analysis of TQI and settlement overruns was carried out, combined with the exploration of under structures such as geo-radar and wave velocity survey, a new service status monitoring method of the high-speed railway subgrade based on the comparison screen of "TQI + settlement value + under-scan" was proposed, while the condition of the subgrade defects was also verified. The results showed that there was no mutual inclusion relation between the overruns of TQI and settlement, and the maximum value didn't correspond to each other. By comparing the frequency and value of TQI overruns, the settlement overruns were more correlated to the frequency. Compared with the previous TQI oriented assessment method, the non-defected conditions with TQI overruns were reduced, and the defected conditions without TQI overruns were increased, which is more consistent with the actual site conditions of high-speed railway. For other in-service high-speed railways which may be greatly affected by the settlement of the soft ground and with a higher proportion of the subgrade, this study can also provide some useful reference for the service status monitoring of them.