The low braking-index pulsar PSR J1734$-$3333 could be born with superhigh internal magnetic fields $B_{rm in}sim10^{15}-10^{16}$ G, and undergo a supercritical accretion soon after its formation in a supernova explosion. The buried multipole magnetic fields will merger into a dipole magnetic field. Since the magnetic flow transfers from the core to the crust of the pulsar, its surface dipole field grows quickly at a power-law form assumed until it saturates at the level of internal dipole field. The increase in surface dipole magnetic field results in the observed low braking index of $n=0.9(2)$. Keeping an average field growth index $varepsilon=1.34(6)$, this pulsar will become a magnetar with surface dipole magnetic field at the equator $B_{rm d}sim 2.6(1)times 10^{14}$,G and $sim 5.3(2)times 10^{14}$,G after next 50,kyrs and 100,kys, respectively.