Abstract: In the PIC method of particle simulations,the physical variables related to the fields are assigned to the grids,while the particles can be anywhere in the simulation domain.Using this method,we design a 2D particle simulation code,and numerically study two common physical phenomena in space physics:beam instability and magnetic reconnection.In the beam instability,the beam plasma streams in the background plasma with the velocity V_b=10V_-A(V_-A is Alfvén speed),and the Alfvén waves are excited through wave-particle interactions.In the magnetic reconnection,the Harris current sheet can induce the magnetic reconnection,and the B_-y shows quadrupolar structure.