Abstract: How to avoid the necking and fusing failures of preformed micro component under the environment of high temperature melt filling flow is a critical scientific problem for the in-mold micro assembly process of polymer micro machine. In this paper, we established a mechanism model of necking and fusing failure for the preformed micro-component induced by high-temperature melt filling flow. The investigation results indicated that the direct driving force of necking and fusing failure for preformed micro-size shaft was derived from its strain softening. Once the strain softening was induced, the necking and fusing damages for preformed micro-size shaft might happen. The strain softening phenomena were controlled by the initial yield stress of preformed micro-size shaft and were negatively correlated with the secondary molding injection temperature. The initial yield stress of preformed PMMA micro-size shaft decreased by 40.1 % from 16.5 MPa to 9.89 MPa when the injection temperature increased from 200 ℃ to 240 ℃. The necking shrinkage of neck section increased from 44.5 % to 70 %. The higher the secondary molding melt injection temperature, the easier was the strain softening and necking fusing failure of preformed micro-size shaft.