The seed solution was synthesized at 100 ℃ using tetrapropylammonium hydroxide as template and commercial silica sol as silicon source. Meanwhile, ZSM-5 zeolites with well crystallinity were successfully synthesized by adding seed solution in the template-free system, and the synthetic conditions of ZSM-5 zeolites were explored. The synthesized zeolites samples were characterized by X-ray powder diffraction (XRD), scanning-electron microscopy (SEM), N2 adsorption-desorption, fourier transform infrared (FT-IR), X-ray fluorescence spectroscopy (XRF) and ammonium temperature-programmed desorption (NH3-TPD). The results show that ZSM-5 zeolites are synthesized by adding only 0.5% (mass fraction) seed solution in the template-free system. Meanwhile, submicron-scale ZSM-5 zeolite aggregates with the size of ~500 nm are synthesized at 170 ℃ for 8 h under optimal synthesis conditions. Furthermore, in the isomerization of trimethylbenzene, the sample with n(SiO2)/n(Al2O3) ratio of 30 has better catalytic performance. Mass conversion of 1, 2, 4-trimethylbenzene, mass yield of 1, 3, 5-trimethylbenzene and selectivity of 1, 3, 5-trimethylbenzene are 3281%, 20.81% and 63.42%, respectively.