Phosphorus (P) and sediment loss through runoff to surface and ground waters represents a risk to human and environmental health. We investigated the characteristics of P loss of maize seedlings on different slope gradients under a simulated rainfall experiment. Surface runoff and sediment were highest on the 20°-slope and lowest on the 15°-slope. The 20°-slope showed least P loss in runoff, which accounted for 57% and 50% of those on 10°- and 15°-slopes, respectively. Available phosphorus (AP) losses in sediment on the 20°-slope were 7.6 and 4.2 times as much as that on the 10°- and 15°-slopes, correspondingly. Subsurface runoff and P losses increased with slope gradients increasing, whereas P loss in subsurface runoff was lower than that in surface runoff. The runoff-sediment-yield demonstrated an increase during rainfall events, whereas P concentration in surface runoff rose and then declined before stabilizing. P form losses increased first and then decreased along with increasing slope gradients. Slope gradients had little influence on AP in sediment. The dissolved total phosphorus (DTP) loss dominated the TP loss in runoff. P losses exhibited a complex relationship with runoff-sediment-yield, and different P-form and each item cannot exist on its own or occur independently. Controlling soil and water losses is necessary to alleviate P losses in slope farmlands.