Following our previous work, we studied the partial eruption of a large-scale horse-shoe-like filament that had been observed in a decaying active region on the solar disk for more than 4.5 days. The filament became active after it was broken into two pieces, P1 and P2 seen in H & alpha;, by magnetic reconnection between the magnetic field around it and that of a newly emerging active region nearby. P1 eventually erupted 13 hr after the breaking and escaped from the Sun, developing to a fast coronal mass ejection, and P2 stayed. But the mass in P1 falling down to P2 in the eruption suggests that the global magnetic fields over P1 and P2 were still connected to each other prior to the eruption. The reconnection process breaking the filament occurred outside the filament, and P1 and P2 were located almost at the same altitude, so the fashion of the filament partial eruption studied here differs from that of the double-decker model and that of reconnection inside the filament. Analyzing the decay indices of the background fields above P1 and P2, n (1) and n (2), showed that the altitude where n (1) exceeds the critical value of n ( c ) = 1.5 for the loss of equilibrium or the torus instability is lower than that where n (2) > n ( c ), and that n (1) > n (2) always holds at all altitudes. Combining this fact with that the eruption occurred 13 hr after filament was broken by reconnection, we conclude that the eruption of P1 was triggered by the loss of equilibrium or the torus instability in the configuration, and magnetic reconnection breaking the filament helped weaken the confinement of the background field on P1, allowing P1 to erupt. Detailed features of the eruption and the corresponding physical scenario were also discussed.