The 15-MA driver is powered by 24 linear-transformer-driver (LTD) modules connected electrically in parallel. The magnetically-insulated-transmission-line (MITL) system of the 15-MA driver adopts a four-level design. It is expected that the primary source delivers a more than 15 MA current to a physics load. The typical one-dimensional steady-state pressure-balance model is adopted to calculate the electron flow current of the outer MITLs of the 15-MA driver after the magnetic insulation has been established. The cathode plasma expansion and the collisional flow electrons are considered on the basis of that model. Multiple designs with different characteristic parameters of the MITL system include the vacuum impedance of the constant-impedance segment of the outer-MITL, the minimum gap of the outer-MITL, and the location of the post-hole convolute (PHC). The flow currents of these designs are calculated in three typical times (1/3 peak load current time, peak load current time, and 5 ns before the Z-pinch stagnation) by establishing the equivalent circuit model of the 15-MA driver. The influences of these characteristic parameters on the electrical pulse transmission and convergence of the 15 MA driver are obtained. The calculation results show that the electron flow current at the end of MITL is greatly affected by the impedance of the end of MITL after the electron flow current has entered into the steady state magnetic insulation. The flow current decreases from 184.7 kA to 106.9 kA, while the load current is reduced by 0.5 MA, as the vacuum impedance increases from 0.42 Ω to 0.84 Ω. This is mainly because the central inductance increases by about 1.43 nH (from 9.94 nH to 11.37 nH). In the time of 5 ns before load stagnation, the flow current decreases from 181.9 kA to 85.1 kA as the minimum gap of the outer-MITL increases from 7.10 mm to 14.00 mm, and the peak load current drops only by about 0.1 MA. The flow current and load current decrease slowly as the location radius of the PHC decreases until the radius decreases to 7.65 mm. The research in this paper is helpful in guiding the structure optimization for the central converging region of future Z-pinch driver.