Spherical CdSxSe1-x alloy semiconductor QDs are receiving incredible research interest due to their composition dependent optical tunability and charge carrier behaviour. These highly luminescent alloy QDs can be used in several applications, due to their very long excited state life time. Herein, we describe synthesis and characterization of highly luminescent (70%) CdSSe alloy QD using XRD, EDX, and HRTEM techniques. Steady state optical absorption and photoluminescence (PL) measurements show the non-linear behaviour of the alloy QDs with changing chalcogenide composition. Time resolved photo luminescence (PL) measurements suggest CdSxSe1-x alloy QD has much higher emission quantum yield (~70%) and long radiative life time (24 ns) as compared to both CdS (CdS = 24%, CdS = 9.5 ns) and CdSe (CdSe = 34%, CdSe = 13.3 ns) QDs. Femtosecond transient absorption measurement has been carried out to unravel charge carrier dynamics in early and late time scale. Electron cooling time for CdS0.7Se0.3 alloy QD found to be extremely slow (Cooling=8 ps) in contrast to both pure CdS (Cooling <100 fs) as well as pure CdSe QD (Cooling=600 fs) due to specially decoupled electron and hole in quasi type II core-shell structure. Charge recombination reaction found to be slowest in alloy QDs as compared to both CdS and CdSe QDs.