Centrifugal blowers are widely used for gas compression in a variety of industrial fields; however, a wider operating range is required in these machines. Investigations on the generation mechanism of unsteady flow (i.e., surge) are very important to improve the operating range. The purpose of this study is to clarify the generation mechanism of pressure fluctuations in a multistage centrifugal blower equipped with inlet guide vanes (IGVs) upstream during the first stage under the IGVs partially open condition. These pressure fluctuations occur at flowrates when the slope of the total system head curve is steeply negative. According to our previous study on the detailed unsteady pressure measurements, this pressure oscillation is supposed to be the mild surge caused by the positive slope of the head curves at the second to the last stages. The slope of the total system head curve was kept negative due to the steeply negative slope of the head curve during the first stage. Thus, the whole compression system seemed to be stable. To confirm the validity of this hypothesis, system dynamic simulations based on Greitzer's lumped-parameter model were conducted using newly measured static pressure-rise characteristic curves of each stage in a four-stage centrifugal blower. In these simulations, the pressure-rise characteristic curves of the first stage and the second to last stages were modeled as two different actuator disks, and the stabilization/destabilization effects of each stage on the system dynamic characteristics were separately taken into account under the IGVs partially open condition. The system dynamic simulation reproduced the mild surge behavior of the system under the IGVs partially open condition when the slope of the total system head curve was still kept steeply negative. The calculated amplitude and frequency of the pressure fluctuations caused by the mild surge showed satisfactory agreement with the measured ones. However, the inception flowrate of the system instability in the simulation was approximately 7% smaller than that in the measurement. From these results, we confirmed that the pressure fluctuation occurred under the IGVs partially open condition was caused by the mild surge due to the positive slope of the pressure-rise characteristic during the second to last stage. In addition, we found that this mild surge was caused by the stall of the vaned diffusers during the second to last stage.