A strong strain hardening ability in ultra-high strength spring steel 60Si2CrVAT was achieved by the design of multiphase microstructures using a quenching-partitioning-tempering process. The microstructure consisted of similar to 70% martensite/bainite and similar to 30% retained austenite, which was obtained at an optimized partitioning temperature of 380 degrees C. The corresponding yield strength ratio and the product of strength and elongation were 0.81 and 34,828 MPa%, respectively. Interestingly, this sample exhibited a double-stage strain hardening characteristic with a concave-down shape, which was distinct from that of the other samples. The excellent performance of the ultra-high strength spring steel was mainly due to the interactions between the multiple phases and the stability of the retained austenite during cold deformation. This paper provides a new way of obtaining a strong strain hardening ability in ultra-high strength steels, which is expected to be applicable to the cold coiling process.