In order to implement the particle-in-cell(PIC) simulation of four outer magnetically insulated transmission lines(MITL) of Z-pinch accelerator, we adopt the backward-going transmission line voltage wave at the double post-hole convolute(DHPC) travelling in the outer MITL direction, which is gained in circuit simulation to model the part from DHPC to Z-pinch load. For the purpose of enhancing the accuracy of equivalent circuit model of DHPC and Z-pinch load, the backward-going transmission line voltage wave that is computed by circuit simulation is rescaled by PIC simulation. The insulator voltage and current waves that are obtained in the PIC simulation of four outer MITLs of PBFA Z can match the electrical measurements better than those obtained by the circuit simulation. Moreover, the influence of Z-pinch load implosion on the process of pulsed power coupling in the outer MILT is analyzed and explained.