<p>This paper is intended to investigate the half-thickness z-pin effect on balancing interlaminar improvement and intralaminar adverse impact of polyimide z-pin reinforced polymer composites. The z-pin pre-hole implanted (ZPI) process was employed to mitigate initial intralaminar damage. The experimental results indicate that z-pins with the different length can significantly improve the mode II fracture toughness (G_II) of specimens. And the reinforced effect of half-thickness z-pins is significantly better than that of full-thickness z-pins, which attribute to the larger bonded area between pulled-out z-pin and laminates. The propagation G_IIC of specimens with a bonded area of 305.36&#xa0;mm² is increased by 524.63%. Compared with unpinned specimens, the flexural strength of specimens with half-thickness z-pins has a retention of 97%. Meanwhile, the plastic strain energy of specimens with half-thickness z-pins is twice as large than that of specimens with full-thickness z-pins. In short, half-thickness z-pins could achieve the desirous equilibrium of mechanical properties between the interlaminar and the intralaminar.</p>