Using the particle swarm optimization algorithm on crystal structure prediction, we first predicted that YAg alloy undergoes a first-order phase transition from CsCl phase to Fd-3m phase at about 25 GPa with a volume collapse of 7.47%. An abrupt change of the electrical resistance was discovered accompanying the phase transformation. Our results showed that external pressure has different effects on the ductility and anisotropy of the two phases. External pressure can improve the ductility of CsCl phase, while it has a negligible effect on that of Fd-3m phase. As pressure increases, the elastic anisotropy of CsCl phase increases rapidly, while that of Fd-3m phase remains unchanged. The structural stability, electronic structure, and thermodynamic properties of the two phases under pressure were also investigated systematically.