Elite parental lines are of great significance to crop breeding. To discover unique genomic loci associated with excellent economic traits in the elite cabbage inbred-line '01-20', we performed comparisons of phenotypes as well as whole-genome insertion-deletion/simple sequence repeat loci between '01-20' and each of its five sister lines. '01-20' has a range of excellent agronomic traits, including early-maturing, and improvements in plant type and leaf colour. Eight unique loci were discovered for '01-20' and '01-07-258', another elite line similar to '01-20' at the whole-genome level. In addition, two excellent double-haploid lines derived from a cross of '01-20' also inherited these loci. Based on the quantitative trait locus association results, five of these loci were found to be associated with important agronomic traits, which could explain why the elite parent '01-20' possesses greener outer leaves, a more compact and upright plant-type, rounder head, shorter core length, and better taste. Additionally, some of these loci have clustering effects for quantitative trait loci associated with different traits; therefore, important genes in these regions were analysed. The obtained results should enable marker-assisted multi-trait selection at the whole-genome level in cabbage breeding and provide insights into significant genome loci and their breeding effects.