Cooling curves and residual deflection during spray quench process are measured by using thermometric instrument and spiral micrometer, separately, in extruded EW75 magnesium alloy. By using Numerical analysis, temperature and stress with the change of time during quenching were also investigated. The results showed that: when the spray speed was 6L/min, the steam film stage was obvious, as the speed reached to 9L/min, steam stage disappeared. The residual deflection increased with the spray speed increased. When the spray speed was 6L/min, the residual deflection was 3.38mm and increased to 3.88mm as the spray speed increased to 9L/min. But with the increase of spray speed, the growth rate of deflection induced. The results of the numerical simulation indicated that during the quench process, the surface cools faster than body and shrink more severely at first, leading stress distribution is compress at surface while tensile in body. Plastic deformation happens at this time. And then along with the surface cooling speed reduced, the body cool speed is greater than surface; the shrink in body is also greater, thus the compress stress turn to be tensile and contrary change take place at surface. With the increasing distance from the cross section, the tensile stress is reduced gradually, in 5.3mm, there exists a zero stress layer, then turns to compressive stress.