The novel up-up-down (UUD) ground state, which has been predicted to be stabilized by quantum fluctuations only, is a good example of exotic magnetism in the triangular-lattice antiferromagnet (TAF). This quantum UUD state, which manifests itself with a constant magnetization (1/3 of the saturation magnetization) over a finite field range, has been reported in the S=1/2 TAF Ba3CoSb2O9. Recently, we present a systematic study on the effect of quenched disorder on Ba2.8Sr0.2CoSb2O9 using DC and AC susceptibilities and advanced neutron scattering. DC susceptibility at zero field confirms an antiferromagnetic ordering below 2.8 K with an easy-plane anisotropy in Ba2.8Sr0.2CoSb2O9. And AC susceptibility measurement suggests that the field extent for the UUD phase of Ba2.8Sr0.2CoSb2O9 is larger than that of Ba3CoSb2O9. We also report the neutron-scattering study on the evolution of the ordering temperature and magnetic structure as a function of applied magnetic field.