From the macroscopic point of view, the plastic zone (PZ) is obtained based on the distributed dislocation technique (DDT) and von Mises yield criterion. From the microscopic point of view, PZ is determined by the DDT model. The effect of micro-cracks on PZ of the macro-crack tip is analyzed. The results show that the micro-crack has a little amplification effect on PZ of the macro-crack tip when it locates in front of PZ. As the micro-crack is close to the macro-crack tip, PZ of the macro-crack tip and the micro-crack tip will join together. When the micro-crack enters into PZ of the macro-crack tip, it has an obvious shielding effect on PZ. When the micro-crack is behind the macro-crack tip, the width of PZ decreases while the height increases. The dislocation distribution in PZ is in the form of inverse pileup. The amplification and shielding regions are divided into five strip-shaped regions, and they appear alternately. The results can provide useful information to predict plastic behaviors near crack tip. The analysis of amplification and shielding effect is important to materials design.