  Recent timing observation reported that the radio pulsar PSR J1734 - 3333
with a rotating period $P=1.17~\rm s$ is slowing down with a period derivative
$\dot{P}=2.28\times 10^{-12}\rm s\,s^{-1}$. Its derived braking index $n=0.9
\pm 0.2$ is the lowest value among young radio pulsars with the measured
braking indices. In this Letter, we attempt to investigate the influence of the
braking torque caused by the interaction between the fall-back disk and the
strong magnetic field of the pulsar on the spin evolution of PSR J1734 - 3333.
Analytical result show that this braking torque is obviously far more than that
by magnetic dipole radiation for pulsars with spin period of $> 0.1$ s, and
play an important role during the spin-down of the pulsars. Our simulated
results indicate that, for some typical neutron star parameters, the braking
index and the period derivative approximately in agreement with the measured
value of PSR J1734 - 3333 if the material inflow rate in the fallback disk is
$2 \times 10^{17} \rm g\,s^{-1}$. In addition, our scenario can account for the
measured braking indices of four young pulsars. However, our predicted X-ray
luminosity are 1 -2 order of magnitude higher than the observation. We proposed
that this discrepancy may originate from the instability of fall-back disk.
