In centrifugal compressors, it is common to observe a rapid reduction in surge margin toward high rotational speed, which shows a non-linear surge characteristic against the change of rotational speed. This paper presents a comprehensive experimental and numerical study to understand the flow mechanisms leading to the non-linear surge behavior in a high-speed centrifugal compressor. It shows that for the studied compressor, there are two critical flow coefficients (ϕ = 0.255 and 0.136) where the stability of the compressor stage is significantly weakened. At a higher speed, the impeller rotating stall happens at the first critical point where the diffuser instability is also enhanced. This is caused by the increase of the flow non-uniformity at the impeller exit as well as increased diffuser inflow angle due to the impeller compressibility effect. Therefore, both the impeller and diffuser’s instability are matched and trigger the surge at a high flow coefficient. In contrast, at a lower speed, the diffuser instability is not enhanced by the impeller rotating stall, this mismatch of the two component’s instability allows the compressor to pass through the first critical point and extend the surge limit to the second critical point where the diffuser reaches the stability limit and causes the rotating stall spontaneously. By these different behaviors at each speed, the non-linearity of the surge characteristic is established.