The breakup of low-viscosity-ratio fluids can often be encountered in the process of CO2 storage through saline aquifer but remains a challenging problem due to its complexity in physics. By using an accurate indirect boundary element method, the breakup of a low-viscosity-ratio isolated drop in a contraction flow at a vanishing Reynolds number is investigated numerically. A practical mathematical method is constructed to detect the asymptotic behavior of maximum curvature at pinching point and used to predict the occurrence of a pinch-off. The 3-D numerical simulation presented here can accurately capture not only the primary breakup of a low-viscosity-ratio drop, but also the secondary breakup and presence of a set of satellite drops. The results agree qualitatively with laboratory experiments but provide more details to understand the process of low-viscosity-ratio drop breakup in a confined outer flow.