The phenomenon of liquid drop impact on wetted spherical surfaces with low impact velocity is observed using a high speed digital camera at 10000 frames per second. Drop rebound, partial rebound and spreading are observed and analyzed, considering the effect of viscosity. Influences of the sphere diameter, impact velocity and viscosity on the spreading characteristic parameter are discussed quantitatively. The experimental observations show that the drop rebound and partial rebound phenomena may occur at large viscosity and low impact velocity, which cannot be observed at small viscosity. The spreading area can be increased by increasing impact velocity. The results also reveal that the spreading factor increases with viscosity decreasing. At the sphere diameters ranging from 4 mm to 20 mm, it is found that with the increase of the sphere diameter, the spreading factor will be increased.