High-resolution SiO observations of the NGC 1333 IRAS 4A star-forming region show a highly collimated outflow with a substantial deflection. The deflection has been suggested to be caused by the interactions of the outflow and a dense cloud core. To investigate the deflection process of protostellar outflows, we carry out three-dimensional hydrodynamic simulations of the collision of an outflow with a dense cloud. Assuming a power-law-type density distribution of the obstructing cloud, the numerical experiments show that the deflection angle is mainly determined by the impact parameter and the density contrast between the outflow and the cloud. The deflection angle is, however, relatively insensitive to the velocity of the outflow. Using a numerical model with physical conditions that are particularly suitable for the IRAS 4A system, we produce a column-density image and a position-velocity diagram along the outflow which are consistent with the observations. Based on our numerical simulations, if we assume that the initial density and the velocity of the outflow are ~10 cm^-3 and ~70 km s^-1, respectively, the densities of the dense core and ambient medium in the IRAS 4A system are most likely to be ~10⁵ cm^-3 and ~10² cm^-3, respectively. We, therefore, demonstrate through numerical simulations that the directional variability of the IRAS 4A outflow can be explained reasonably well using the collision model.