Abstract: In the study of beam dynamics of high intensity linear accelerators, full 3D multiparticle simulations are needed due to numerous nonlinear effects caused by the strong space-charge effects between charged particles. However, the use of multiparticle simulations is limited by the speed of the Particle-in-cell (PIC) algorithm. In this work, we modify the PIC algorithm by using the symmetry of beam, the solution procedure is divided into two steps, one is the solution of the symmetric field and the other is the addition of the asymmetric field components. Due to the full use of symmetry, our modified algorithm can significantly improve the computational speed of multiparticle simulations. Results show that the modified algorithm accelerates the process of solving space-charge field by a factor of 4, thus increases the speed of the entire multiparticle simulation by a factor of 1.7, while ensures the accuracy of the simulation. Although our algorithm is modified for linear accelerator, considering that the algorithm is an improvement based on symmetry, it can still speed up the simulation for any accelerator with symmetry.