Plasma immersion ion implantation (PIII) of the square target with finite length is simulated using a three-dimensional particle-in-cell (PIC) plasma simulation in this paper. The incident dose, the impact angle and the implanted energy on the target surface are investigated. The results show that the sheath around the square target with finite length becomes spherical rapidly during PIII. And the three-dimensional sheath width is small apparently compared with the one simulated by two-dimensional PIC. And it is found that the three-dimensional ion dose is not evenly distributed on the target surface during simulation time (50-1pi) in this work. The dose is smallest in the center of the target, and it is largest near the corner. This is due to spherical sheath where ions are focused and accelerated into near the corner. In the central zone, the ion incidence is nearly normal to the surface, and the impact average energy exceeds 90% of the maximum. But the impact angle near the corner is always nearly 45, and the implanted energy is only about 50% of the maximum.